diff --git a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
index 54a4fe7..1926b96 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -63,7 +63,9 @@ protected function setUp() {
     $conf = array();
     drupal_static_reset();
 
-    // Collect and set fixed module list.
+    // Ensure that the module list is initially empty.
+    $this->moduleList = array();
+    // Collect and set a fixed module list.
     $class = get_class($this);
     $modules = array();
     while ($class) {
@@ -72,13 +74,9 @@ protected function setUp() {
       }
       $class = get_parent_class($class);
     }
-    foreach ($modules as $module) {
-      $this->moduleList[$module] = drupal_get_path('module', $module);
-    }
-    module_list(NULL, $this->moduleList);
-    module_load_all();
+    $this->enableModules($modules, FALSE);
 
-    // Provide a minimal environment for unit tests.
+    // Provide a minimal, partially mocked environment for unit tests.
     $conf['file_public_path'] = $this->public_files_directory;
     $conf['lock_backend'] = 'Drupal\Core\Lock\NullLockBackend';
     $conf['cache_classes'] = array('cache' => 'Drupal\Core\Cache\NullBackend');
@@ -118,25 +116,40 @@ protected function installSchema($module, $table) {
   /**
    * Enables modules for this test.
    *
-   * The {system} table is required for using this method. Install it via:
+   * Callbacks invoked by module_enable() may need to access information
+   * provided by info hooks of the new modules already. However, module_enable()
+   * enables the new modules in {system} only, but that has no effect, since we
+   * are operating with a fixed module list.
+   *
+   * The {system} table is required to install new modules. Install it via:
    * @code
-   * $this->installSchema('system', 'system');
+   *   $this->installSchema('system', 'system');
    * @endcode
    *
    * @param array $modules
    *   A list of modules to enable.
+   * @param bool $install
+   *   (optional) Whether to install the list of modules via module_enable().
+   *   Defaults to TRUE. Requires the {system} table to exist. If FALSE, the new
+   *   modules are only added to the fixed module list and loaded.
+   *
+   * @todo Remove this method as soon as there is an Extensions service
+   *   implementation that is able to manage a fixed module list.
    */
-  protected function enableModules(array $modules) {
-    // Add the new modules to the fixed module_list() first, since callbacks
-    // invoked by module_enable() may need to access information provided by
-    // info hooks of the new modules already. module_enable() enables the new
-    // modules in {system}, but that has no effect, since we are operating with
-    // a fixed module list.
+  protected function enableModules(array $modules, $install = TRUE) {
+    // Set the modules in the fixed module_list().
     foreach ($modules as $module) {
       $this->moduleList[$module] = drupal_get_path('module', $module);
     }
     module_list(NULL, $this->moduleList);
 
-    module_enable($modules);
+    // Call module_enable() to enable (install) the new modules.
+    if ($install) {
+      module_enable($modules);
+    }
+    // Otherwise, only ensure that the new modules are loaded.
+    else {
+      module_load_all(FALSE, TRUE);
+    }
   }
 }