diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 2b45c64..951bd55 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -28,48 +28,16 @@
   </php>
   <testsuites>
     <testsuite name="unit">
-      <directory>./tests/Drupal/Tests</directory>
-      <directory>./modules/*/tests/src/Unit</directory>
-      <directory>../modules/*/tests/src/Unit</directory>
-      <directory>../profiles/*/tests/src/Unit</directory>
-      <directory>../sites/*/modules/*/tests/src/Unit</directory>
-      <!-- Exclude Composer's vendor directory so we don't run tests there. -->
-      <exclude>./vendor</exclude>
-      <!-- Exclude Drush tests. -->
-      <exclude>./drush/tests</exclude>
+      <file>./tests/TestSuites/UnitTestSuite.php</file>
     </testsuite>
     <testsuite name="kernel">
-      <directory>./tests/Drupal/KernelTests</directory>
-      <directory>./modules/*/tests/src/Kernel</directory>
-      <directory>../modules/*/tests/src/Kernel</directory>
-      <directory>../profiles/*/tests/src/Kernel</directory>
-      <directory>../sites/*/modules/*/tests/src/Kernel</directory>
-      <!-- Exclude Composer's vendor directory so we don't run tests there. -->
-      <exclude>./vendor</exclude>
-      <!-- Exclude Drush tests. -->
-      <exclude>./drush/tests</exclude>
+      <file>./tests/TestSuites/KernelTestSuite.php</file>
     </testsuite>
     <testsuite name="functional">
-      <directory>./tests/Drupal/FunctionalTests</directory>
-      <directory>./modules/*/tests/src/Functional</directory>
-      <directory>../modules/*/tests/src/Functional</directory>
-      <directory>../profiles/*/tests/src/Functional</directory>
-      <directory>../sites/*/modules/*/tests/src/Functional</directory>
-      <!-- Exclude Composer's vendor directory so we don't run tests there. -->
-      <exclude>./vendor</exclude>
-      <!-- Exclude Drush tests. -->
-      <exclude>./drush/tests</exclude>
+      <file>./tests/TestSuites/FunctionalTestSuite.php</file>
     </testsuite>
     <testsuite name="functional-javascript">
-      <directory>./tests/Drupal/FunctionalJavascriptTests</directory>
-      <directory>./modules/*/tests/src/FunctionalJavascript</directory>
-      <directory>../modules/*/tests/src/FunctionalJavascript</directory>
-      <directory>../profiles/*/tests/src/FunctionalJavascript</directory>
-      <directory>../sites/*/modules/*/tests/src/FunctionalJavascript</directory>
-      <!-- Exclude Composer's vendor directory so we don't run tests there. -->
-      <exclude>./vendor</exclude>
-      <!-- Exclude Drush tests. -->
-      <exclude>./drush/tests</exclude>
+      <file>./tests/TestSuites/FunctionalJavascriptTestSuite.php</file>
     </testsuite>
   </testsuites>
   <listeners>
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php b/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
index 834d0a4..7c5e855 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
@@ -22,7 +22,7 @@
 /**
  * Base class for the actual unit tests testing \Drupal\Core\Render\Renderer.
  */
-class RendererTestBase extends UnitTestCase {
+abstract class RendererTestBase extends UnitTestCase {
 
   /**
    * The tested renderer.
diff --git a/core/tests/TestSuites/FunctionalJavascriptTestSuite.php b/core/tests/TestSuites/FunctionalJavascriptTestSuite.php
new file mode 100644
index 0000000..feb7b0c
--- /dev/null
+++ b/core/tests/TestSuites/FunctionalJavascriptTestSuite.php
@@ -0,0 +1,61 @@
+<?php
+
+namespace Drupal\Tests\TestSuites;
+
+use Drupal\simpletest\TestDiscovery;
+use Drupal\Tests\TestSuites\TestSuiteBase;
+
+require_once __DIR__ . '/TestSuiteBase.php';
+
+/**
+ * Discovers tests for the unit test suite.
+ */
+class FunctionalJavascriptTestSuite extends TestSuiteBase {
+
+  /**
+   * Factory method which loads up a suite with all functional javascript tests.
+   *
+   * @return static
+   *   The test suite.
+   */
+  public static function suite() {
+    $root = dirname(dirname(dirname(__DIR__)));
+
+    $suite = new self('functional-javascript');
+    $suite->addTestFiles(static::getFunctionalJavascriptTests($root));
+    return $suite;
+  }
+
+  /**
+   * Finds the functional javascript tests.
+   *
+   * @param string $root
+   *   Path to the root of the Drupal installation.
+   *
+   * @return string[]
+   *   Classmap of file paths for tests, keyed by fully-qualified name.
+   */
+  protected static function getFunctionalJavascriptTests($root = '') {
+    // Core's functional javascript tests are in the namespace
+    // Drupal\FunctionalJavascriptTests\ and are always inside of
+    // core/tests/Drupal/FunctionalJavascriptTests.
+    $classmap = TestDiscovery::scanDirectory('Drupal\\FunctionalJavascriptTests\\', $root . '/core/tests/Drupal/FunctionalJavascriptTests');
+
+    // Extensions' functional javascript tests will always be in the namespace
+    // Drupal\Tests\extension-name\FunctionalJavascript\ and be in the
+    // extension-path/tests/src/FunctionalJavascript directory. Not all
+    // extensions will have functional javascript tests.
+    foreach (static::findExtensionDirectories($root) as $extension_name => $dir) {
+      $test_path = $dir . '/tests/src/FunctionalJavascript';
+      if (is_dir($test_path)) {
+        $classmap = array_merge(
+          TestDiscovery::scanDirectory("Drupal\\Tests\\$extension_name\\FunctionalJavascript\\", $test_path),
+          $classmap
+        );
+      }
+    }
+
+    return $classmap;
+  }
+
+}
diff --git a/core/tests/TestSuites/FunctionalTestSuite.php b/core/tests/TestSuites/FunctionalTestSuite.php
new file mode 100644
index 0000000..1f380e3
--- /dev/null
+++ b/core/tests/TestSuites/FunctionalTestSuite.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\Tests\TestSuites;
+
+use Drupal\simpletest\TestDiscovery;
+use Drupal\Tests\TestSuites\TestSuiteBase;
+
+require_once __DIR__ . '/TestSuiteBase.php';
+
+/**
+ * Discovers tests for the unit test suite.
+ */
+class FunctionalTestSuite extends TestSuiteBase {
+
+  /**
+   * Factory method which loads up a suite with all unit tests.
+   *
+   * @return self
+   *   The test suite.
+   */
+  public static function suite() {
+    $root = dirname(dirname(dirname(__DIR__)));
+
+    $suite = new self('functional');
+    $suite->addTestFiles(static::getFunctionalTests($root));
+    return $suite;
+  }
+
+  /**
+   * Finds the functional tests.
+   *
+   * @param string $root
+   *   Path to the root of the Drupal installation.
+   *
+   * @return string[]
+   *   Classmap of file paths for tests, keyed by fully-qualified name.
+   */
+  protected static function getFunctionalTests($root = '') {
+    // Core's functional tests are in the namespace Drupal\FunctionalTests\ and
+    // are always inside of core/tests/Drupal/FunctionalTests.
+    $classmap = TestDiscovery::scanDirectory('Drupal\\FunctionalTests\\', $root . '/core/tests/Drupal/FunctionalTests');
+
+    // Extensions' functional tests will always be in the namespace
+    // Drupal\Tests\extension-name\Functional\ and be in the
+    // extension-path/tests/src/Functional directory. Not all extensions will
+    // have kernel tests.
+    foreach (static::findExtensionDirectories() as $extension_name => $dir) {
+      $test_path = $dir . '/tests/src/Functional';
+      if (is_dir($test_path)) {
+        $classmap = array_merge(
+          TestDiscovery::scanDirectory("Drupal\\Tests\\$extension_name\\Functional\\", $test_path),
+          $classmap
+        );
+      }
+    }
+
+    return $classmap;
+  }
+
+}
diff --git a/core/tests/TestSuites/KernelTestSuite.php b/core/tests/TestSuites/KernelTestSuite.php
new file mode 100644
index 0000000..effe555
--- /dev/null
+++ b/core/tests/TestSuites/KernelTestSuite.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\Tests\TestSuites;
+
+use Drupal\simpletest\TestDiscovery;
+use Drupal\Tests\TestSuites\TestSuiteBase;
+
+require_once __DIR__ . '/TestSuiteBase.php';
+
+/**
+ * Discovers tests for the kernel test suite.
+ */
+class KernelTestSuite extends TestSuiteBase {
+
+  /**
+   * Factory method which loads up a suite with all unit tests.
+   *
+   * @return self
+   *   The test suite.
+   */
+  public static function suite() {
+    $root = dirname(dirname(dirname(__DIR__)));
+
+    $suite = new self('kernel');
+    $suite->addTestFiles(static::getKernelTests($root));
+    return $suite;
+  }
+
+  /**
+   * Finds the kernel tests.
+   *
+   * @param string $root
+   *   Path to the root of the Drupal installation.
+   *
+   * @return string[]
+   *   Classmap of file paths for tests, keyed by fully-qualified name.
+   */
+  protected static function getKernelTests($root = '') {
+    // Core's kernel tests are in the namespace Drupal\KernelTests\ and are
+    // always inside of core/tests/Drupal/KernelTests.
+    $classmap = TestDiscovery::scanDirectory('Drupal\\KernelTests\\', $root . '/core/tests/Drupal/KernelTests');
+
+    // Extensions' kernel tests will always be in the namespace
+    // Drupal\Tests\extension-name\Kernel\ and be in the
+    // extension-path/tests/src/Kernel directory. Not all extensions will have
+    // kernel tests.
+    foreach (static::findExtensionDirectories() as $extension_name => $dir) {
+      $test_path = $dir . '/tests/src/Kernel';
+      if (is_dir($test_path)) {
+        $classmap = array_merge(
+          TestDiscovery::scanDirectory("Drupal\\Tests\\$extension_name\\Kernel\\", $test_path),
+          $classmap
+        );
+      }
+    }
+
+    return $classmap;
+  }
+
+}
diff --git a/core/tests/TestSuites/TestSuiteBase.php b/core/tests/TestSuites/TestSuiteBase.php
new file mode 100644
index 0000000..b924010
--- /dev/null
+++ b/core/tests/TestSuites/TestSuiteBase.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\Tests\TestSuites;
+
+/**
+ * Base class for Drupal test suites.
+ */
+class TestSuiteBase extends \PHPUnit_Framework_TestSuite {
+
+  /**
+   * Finds extensions in a Drupal installation.
+   *
+   * An extension is defined as a directory with an *.info.yml file in it.
+   *
+   * @param string $root
+   *   Path to the root of the Drupal installation to scan.
+   *
+   * @return string[]
+   *   Associative array of extension paths, with extension name as keys.
+   *
+   * @todo Allow injecting root into
+   *   drupal_phpunit_contrib_extension_directory_roots() so that we can be sure
+   *   we're scanning the same directory as the rest of the suite loaders.
+   */
+  public static function findExtensionDirectories() {
+    $extension_roots = \drupal_phpunit_contrib_extension_directory_roots();
+
+    $extension_directories = array_map('drupal_phpunit_find_extension_directories', $extension_roots);
+    return array_reduce($extension_directories, 'array_merge', array());
+  }
+
+}
diff --git a/core/tests/TestSuites/UnitTestSuite.php b/core/tests/TestSuites/UnitTestSuite.php
new file mode 100644
index 0000000..23fb7df
--- /dev/null
+++ b/core/tests/TestSuites/UnitTestSuite.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\Tests\TestSuites;
+
+use Drupal\simpletest\TestDiscovery;
+use Drupal\Tests\TestSuites\TestSuiteBase;
+
+require_once __DIR__ . '/TestSuiteBase.php';
+
+/**
+ * Discovers tests for the unit test suite.
+ */
+class UnitTestSuite extends TestSuiteBase {
+
+  /**
+   * Factory method which loads up a suite with all unit tests.
+   *
+   * @return static
+   *   The test suite.
+   */
+  public static function suite() {
+    $root = dirname(dirname(dirname(__DIR__)));
+
+    $suite = new self('unit');
+    $suite->addTestFiles(static::getUnitTests($root));
+    return $suite;
+  }
+
+  /**
+   * Finds the unit tests.
+   *
+   * @param string $root
+   *   Path to the root of the Drupal installation.
+   *
+   * @return string[]
+   *   Classmap of file paths for tests, keyed by fully-qualified name.
+   */
+  protected static function getUnitTests($root = '') {
+    // Core's unit tests are in the namespace Drupal\Tests\ and are always
+    // inside of core/tests/Drupal/Tests.
+    $classmap = TestDiscovery::scanDirectory('Drupal\\Tests\\', $root . '/core/tests/Drupal/Tests');
+
+    // Extensions' unit tests will always be in the namespace
+    // Drupal\Tests\extension-name\Unit\ and be in the
+    // extension-path/tests/src/Unit directory. Not all extensions will have
+    // unit tests.
+    foreach (static::findExtensionDirectories() as $extension_name => $dir) {
+      $test_path = $dir . '/tests/src/Unit';
+      if (is_dir($test_path)) {
+        $classmap = array_merge(
+          TestDiscovery::scanDirectory("Drupal\\Tests\\$extension_name\\Unit\\", $test_path),
+          $classmap
+        );
+      }
+    }
+
+    return $classmap;
+  }
+
+}