diff --git a/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php b/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
index bebb431..d545172 100644
--- a/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
+++ b/core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\TestSuites;
 
+use Drupal\Tests\TestSuites\TestSuiteBase;
 use org\bovigo\vfs\vfsStream;
 
 /**
@@ -33,6 +34,18 @@ protected function getFilesystem() {
           ],
         ],
       ],
+      'modules' => [
+        'test_extension' => [
+          'test_extension.info.yml' => 'type: module',
+          'tests' => [
+            'src' => [
+              'NotUnit' => [
+                'ContribNotUnitTest.php' => '<?php',
+              ],
+            ],
+          ],
+        ],
+      ],
     ];
   }
 
@@ -86,8 +99,55 @@ public function testAddTestsBySuiteNamespaceCore($filesystem, $suite_namespace,
     $ref_add_tests->invokeArgs($stub, [vfsStream::url('root'), $suite_namespace]);
 
     // Determine if we loaded the expected test files.
-    $this->assertNotEmpty($stub->testFiles);
-    $this->assertEmpty(array_diff_assoc($expected_tests, $stub->testFiles));
+    $this->assertSame($expected_tests, $stub->testFiles);
+  }
+
+  public function provideContribTests() {
+    $filesystem = $this->getFilesystem();
+    return [
+      'contrib-not-unit' => [
+        $filesystem,
+        'NotUnit',
+        [
+          'test_extension' => 'vfs://root/modules/test_extension',
+        ],
+        [
+          'Drupal\NotUnitTests\CoreNotUnitTest' => 'vfs://root/core/tests/Drupal/NotUnitTests/CoreNotUnitTest.php',
+          'Drupal\Tests\test_extension\NotUnit\ContribNotUnitTest' => 'vfs://root/modules/test_extension/tests/src/NotUnit/ContribNotUnitTest.php',
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests for special case behavior of unit test suite namespaces in core.
+   *
+   * @covers ::addTestsBySuiteNamespace
+   *
+   * @dataProvider provideContribTests
+   */
+  public function testAddTestsBySuiteNamespaceContrib($filesystem, $suite_namespace, $discovered_extension_tests, $expected_tests) {
+    // Set up the file system.
+    $vfs = vfsStream::setup('root');
+    vfsStream::create($filesystem, $vfs);
+
+    // Make a stub suite base to test.
+    $stub = new StubTestSuiteBase('test_me');
+
+    // Set up our mock extension directories. We have to stub out
+    // findExtensionDirectories() because we can't inject a vfsStream file
+    // system.
+    $stub->extensionDirectories = $discovered_extension_tests;
+
+    // Access addTestsBySuiteNamespace().
+    $ref_add_tests = new \ReflectionMethod($stub, 'addTestsBySuiteNamespace');
+    $ref_add_tests->setAccessible(TRUE);
+
+    // Invoke addTestsBySuiteNamespace().
+    $ref_add_tests->invokeArgs($stub, [vfsStream::url('root'), $suite_namespace]);
+
+    // Determine if we loaded the expected test files.
+    $this->assertSame($expected_tests, $stub->testFiles);
   }
 
 }
@@ -108,6 +168,13 @@ class StubTestSuiteBase extends TestSuiteBase {
   public $testFiles = [];
 
   /**
+   * Extension directories.
+   *
+   * @var string[]
+   */
+  public $extensionDirectories = [];
+
+  /**
    * {@inheritdoc}
    */
   protected function findExtensionDirectories($root) {
@@ -116,7 +183,7 @@ protected function findExtensionDirectories($root) {
     // which uses \SplFileInfo->getRealPath(). getRealPath() resolves
     // stream-based paths to an empty string. See
     // https://github.com/mikey179/vfsStream/wiki/Known-Issues
-    return [];
+    return $this->extensionDirectories;
   }
 
   /**