Issue #1974266 by ParisLiakos: Register test module classes to PHPunit.

core/modules/simpletest/tests/Drupal/simpletest/Tests/PhpUnitAutoloaderTest.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\simpletest\Tests\PhpUnitAutoloaderTest.
+ */
+
+namespace Drupal\simpletest\Tests;
+
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Test PHPUnit autoloader works correctly.
+ */
+class PhpUnitAutoloaderTest extends UnitTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'PHPUnit autoloader',
+      'description' => 'Test that classes are correctly loaded during PHPUnit initialization.',
+      'group' => 'Simpletest',
+    );
+  }
+
+  /**
+   * Test loading of classes provided by test sub modules.
+   */
+  public function testPhpUnitTestClassesLoading() {
+    $this->assertTrue(class_exists('\Drupal\phpunit_test\PhpUnitTestDummyClass'), 'Class provided by test module was not autoloaded.');
+  }
+
+}

core/modules/simpletest/tests/modules/phpunit_test/lib/Drupal/phpunit_test/PhpUnitTestDummyClass.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\phpunit_test\PhpUnitTestDummyClass.
+ */
+
+namespace Drupal\phpunit_test;
+
+class PhpUnitTestDummyClass {
+}

core/tests/bootstrap.php

@@ -8,6 +8,13 @@
 
 foreach (scandir(__DIR__ . "/../modules") as $module) {
   $loader->add('Drupal\\' . $module, __DIR__ . "/../modules/" . $module . "/lib");
+  // Add test module classes.
+  $test_modules_dir = __DIR__ . "/../modules/$module/tests/modules";
+  if (is_dir($test_modules_dir)) {
+    foreach (scandir($test_modules_dir) as $test_module) {
+      $loader->add('Drupal\\' . $test_module, $test_modules_dir . '/' . $test_module . '/lib');
+    }
+  }
 }
 
 require __DIR__ . "/../../core/lib/Drupal.php";