diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index 903a64a..8b3573d 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -103,6 +103,11 @@ public function registerTestNamespaces() {
     $this->testNamespaces['Drupal\\FunctionalTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalTests'];
     $this->testNamespaces['Drupal\\FunctionalJavascriptTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalJavascriptTests'];
 
+    // Add core namespaces to autoloader.
+    foreach ($this->testNamespaces as $prefix => $paths) {
+      $this->classLoader->addPsr4($prefix, $paths);
+    }
+
     $this->availableExtensions = array();
     foreach ($this->getExtensions() as $name => $extension) {
       $this->availableExtensions[$extension->getType()][$name] = $name;
@@ -116,12 +121,17 @@ public function registerTestNamespaces() {
       // Add Simpletest test namespace.
       $this->testNamespaces["Drupal\\$name\\Tests\\"][] = "$base_path/src/Tests";
 
-      // Add PHPUnit-based test namespaces.
-      $this->testNamespaces["Drupal\\Tests\\$name\\"][] = "$base_path/tests/src";
-    }
-
-    foreach ($this->testNamespaces as $prefix => $paths) {
-      $this->classLoader->addPsr4($prefix, $paths);
+      // Add PHPUnit test suite namespaces.
+      $this->testNamespaces["Drupal\\Tests\\$name\\Unit\\"][] = "$base_path/tests/src/Unit";
+      $this->testNamespaces["Drupal\\Tests\\$name\\Kernel\\"][] = "$base_path/tests/src/Kernel";
+      $this->testNamespaces["Drupal\\Tests\\$name\\Functional\\"][] = "$base_path/tests/src/Functional";
+      $this->testNamespaces["Drupal\\Tests\\$name\\FunctionalJavascript\\"][] = "$base_path/tests/src/FunctionalJavascript";
+
+      // Register module namespace with classloader. We support loading from a
+      // more lenient namespace for traits and helpers.
+      // @see drupal_phpunit_populate_class_loader()
+      // @see drupal_phpunit_get_extension_namespaces()
+      $this->classLoader->addPsr4("Drupal\\Tests\\$name\\", "$base_path/tests/src");
     }
 
     return $this->testNamespaces;
@@ -438,8 +448,8 @@ public static function parseTestClassAnnotations(\ReflectionClass $class) {
   /**
    * Determines the phpunit testsuite for a given classname.
    *
-   * The current PHPUnit test suites are: Unit, Kernel, Functional,
-   * FunctionalJavascript, and Unclassified.
+   * The current PHPUnit test suites are: Unit, Kernel, Functional, and
+   * FunctionalJavascript
    *
    * @param string $classname
    *   The test classname.
@@ -461,9 +471,7 @@ public static function getPhpunitTestSuite($classname) {
       if (in_array($matches[2], ['Unit', 'Kernel', 'Functional', 'FunctionalJavascript'])) {
         return $matches[2];
       }
-      // If there is no match with the defined suites fallback to 'Unclassified'
-      // as all tests must have a non-empty value for a test suite.
-      return 'Unclassified';
+      return FALSE;
     }
     // Core tests.
     elseif (preg_match('/Drupal\\\\(\w*)Tests\\\\/', $classname, $matches)) {
diff --git a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
index a7dd062..2dbb8b7 100644
--- a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
+++ b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
@@ -127,11 +127,15 @@ public function testRegisterTestNamespaces() {
     // the classloader if the module is not 'enabled'.
     $src_namespace = 'Drupal\\test_module\\';
     $this->assertArrayHasKey($src_namespace, $classloader_psr4);
+    $this->assertArrayHasKey('Drupal\\Tests\\test_module\\', $classloader_psr4);
     $this->assertFalse(isset($discovery_psr4[$src_namespace]));
 
     $test_namespaces = [
       'Drupal\\test_module\\Tests\\',
-      'Drupal\\Tests\\test_module\\',
+      'Drupal\\Tests\\test_module\\Unit\\',
+      'Drupal\\Tests\\test_module\\Kernel\\',
+      'Drupal\\Tests\\test_module\\Functional\\',
+      'Drupal\\Tests\\test_module\\FunctionalJavascript\\',
     ];
     foreach ($test_namespaces as $test_namespace) {
       $this->assertArrayHasKey($test_namespace, $discovery_psr4);
@@ -168,7 +172,6 @@ public function testGetTestClasses() {
       'Drupal\Tests\test_module\Functional\FunctionalExampleTest2' => 'PHPUnit-Functional',
       'Drupal\Tests\test_module\Kernel\KernelExampleTest' => 'PHPUnit-Kernel',
       'Drupal\Tests\test_module\FunctionalJavascript\FunctionalJavascriptTest' => 'PHPUnit-FunctionalJavascript',
-      'Drupal\Tests\test_module\Integration\IntegrationTest' => 'PHPUnit-Unclassified',
       'Drupal\Tests\test_module\Unit\UnitTest' => 'PHPUnit-Unit',
     ];
     foreach ($expected_classes as $expected_class => $expected_testsuite) {
@@ -220,7 +223,7 @@ public function providerTestGetPhpunitTestSuite() {
     ];
     $data['module-unclassified'] = [
       '\Drupal\Tests\rules\Integration\RulesIntegrationTest',
-      'Unclassified',
+      FALSE,
     ];
     $data['core-unittest'] = [
       '\Drupal\Tests\ComposerIntegrationTest',
diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 507ee16..2b45c64 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -71,33 +71,6 @@
       <!-- Exclude Drush tests. -->
       <exclude>./drush/tests</exclude>
     </testsuite>
-    <testsuite name="unclassified">
-      <directory>./modules/*/tests/src</directory>
-      <directory>../modules/*/tests/src</directory>
-      <directory>../profiles/*/tests/src</directory>
-      <directory>../sites/*/modules/*/tests/src</directory>
-      <!-- Exclude Composer's vendor directory so we don't run tests there. -->
-      <exclude>./vendor</exclude>
-      <!-- Exclude Drush tests. -->
-      <exclude>./drush/tests</exclude>
-      <!-- Exclude special test types -->
-      <exclude>./modules/*/tests/src/Unit</exclude>
-      <exclude>../modules/*/tests/src/Unit</exclude>
-      <exclude>../profiles/*/tests/src/Unit</exclude>
-      <exclude>../sites/*/modules/*/tests/src/Unit</exclude>
-      <exclude>./modules/*/tests/src/Kernel</exclude>
-      <exclude>../modules/*/tests/src/Kernel</exclude>
-      <exclude>../profiles/*/tests/src/Kernel</exclude>
-      <exclude>../sites/*/modules/*/tests/src/Kernel</exclude>
-      <exclude>./modules/*/tests/src/Functional</exclude>
-      <exclude>../modules/*/tests/src/Functional</exclude>
-      <exclude>../profiles/*/tests/src/Functional</exclude>
-      <exclude>../sites/*/modules/*/tests/src/Functional</exclude>
-      <exclude>./modules/*/tests/src/FunctionalJavascript</exclude>
-      <exclude>../modules/*/tests/src/FunctionalJavascript</exclude>
-      <exclude>../profiles/*/tests/src/FunctionalJavascript</exclude>
-      <exclude>../sites/*/modules/*/tests/src/FunctionalJavascript</exclude>
-    </testsuite>
   </testsuites>
   <listeners>
     <listener class="\Drupal\Tests\Listeners\DrupalStandardsListener">