diff --git a/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php b/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php
index 445c225e8b..5a38c89832 100644
--- a/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php
+++ b/core/tests/Drupal/Tests/Core/Test/TestSuiteBaseTest.php
@@ -6,17 +6,22 @@
 use org\bovigo\vfs\vfsStream;
 use PHPUnit\Framework\TestCase;
 
-// Require the base class. We do this to avoid making the test suite classes
-// autoloadable.
-require_once __DIR__ . '/../../TestSuites/TestSuiteBase.php';
-
 /**
  * @coversDefaultClass \Drupal\TestSuites\TestSuiteBase
  *
+ * @runTestsInSeparateProcesses
+ *
  * @group TestSuite
  */
 class TestSuiteBaseTest extends TestCase {
 
+  protected function setUp() {
+    parent::setUp();
+    // Require the base class. We do this to avoid making the test suite classes
+    // autoloadable.
+    require_once __DIR__ . '/../../../TestSuites/TestSuiteBase.php';
+  }
+
   /**
    * Helper method to set up the file system.
    *
@@ -88,18 +93,43 @@ public function testAddTestsBySuiteNamespaceCore($filesystem, $suite_namespace,
     $vfs = vfsStream::setup('root');
     vfsStream::create($filesystem, $vfs);
 
-    // Make a stub suite base to test.
-    $stub = new StubTestSuiteBase('test_me');
+    $mock = $this->getMockBuilder(TestSuiteBase::class)
+      ->setConstructorArgs(['test_me'])
+      ->setMethods(['findExtensionDirectories', 'addTestFiles'])
+      ->getMockForAbstractClass();
+
+    // The test files discovered by our mock.
+    $test_files = [];
+
+    // We have to stub findExtensionDirectories() because we can't inject a
+    // vfsStream filesystem into drupal_phpunit_find_extension_directories(),
+    // which uses \SplFileInfo->getRealPath(). getRealPath() resolves
+    // stream-based paths to an empty string. See
+    // https://github.com/mikey179/vfsStream/wiki/Known-Issues
+    $mock->expects($this->once())
+      ->method('findExtensionDirectories')
+      ->willReturn([]);
+    // We mock addTestFiles() because the parent implementation can't deal with
+    // vfsStream-based filesystems due to an error in
+    // stream_resolve_include_path(). See
+    // https://github.com/mikey179/vfsStream/issues/5 Here we just store the
+    // test file being added in $test_files.
+     $mock->expects($this->once())
+      ->method('addTestFiles')
+      ->willReturnCallback(
+        function ($filenames) use (&$test_files) {
+          $test_files = array_merge($test_files, $filenames);
+      });
 
     // Access addTestsBySuiteNamespace().
-    $ref_add_tests = new \ReflectionMethod($stub, 'addTestsBySuiteNamespace');
+    $ref_add_tests = new \ReflectionMethod($mock, 'addTestsBySuiteNamespace');
     $ref_add_tests->setAccessible(TRUE);
 
     // Invoke addTestsBySuiteNamespace().
-    $ref_add_tests->invokeArgs($stub, [vfsStream::url('root'), $suite_namespace]);
+    $ref_add_tests->invokeArgs($mock, [vfsStream::url('root'), $suite_namespace]);
 
     // Determine if we loaded the expected test files.
-    $this->assertEquals($expected_tests, $stub->testFiles);
+    $this->assertEquals($expected_tests, $test_files);
   }
 
   /**
@@ -111,44 +141,3 @@ public function testLocalTimeZone() {
   }
 
 }
-
-/**
- * Stub subclass of TestSuiteBase.
- *
- * We use this class to alter the behavior of TestSuiteBase so it can be
- * testable.
- */
-class StubTestSuiteBase extends TestSuiteBase {
-
-  /**
-   * Test files discovered by addTestsBySuiteNamespace().
-   *
-   * @var string[]
-   */
-  public $testFiles = [];
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function findExtensionDirectories($root) {
-    // We have to stub findExtensionDirectories() because we can't inject a
-    // vfsStream filesystem into drupal_phpunit_find_extension_directories(),
-    // which uses \SplFileInfo->getRealPath(). getRealPath() resolves
-    // stream-based paths to an empty string. See
-    // https://github.com/mikey179/vfsStream/wiki/Known-Issues
-    return [];
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function addTestFiles($filenames) {
-    // We stub addTestFiles() because the parent implementation can't deal with
-    // vfsStream-based filesystems due to an error in
-    // stream_resolve_include_path(). See
-    // https://github.com/mikey179/vfsStream/issues/5 Here we just store the
-    // test file being added in $this->testFiles.
-    $this->testFiles = array_merge($this->testFiles, $filenames);
-  }
-
-}