diff -u b/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
--- b/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -134,6 +134,9 @@
   // Clear out the previous verbose files.
   file_unmanaged_delete_recursive('public://simpletest/verbose');
 
+  // Prime module data.
+  simpletest_prepare_module_data();
+
   // Get the info for the first test being run.
   $first_test = array_shift($test_list);
   $first_instance = new $first_test();
@@ -388,15 +391,19 @@
  */
 function simpletest_classloader_register($module_data = NULL) {
   if (!$module_data) {
-    $module_data = system_rebuild_module_data();
+    // This is only needed to register namespaces and it is safe to
+    // re-register existing namespaces so go with raw data.
+    $module_data = _system_rebuild_module_data('simpletest_module_data');
   }
   foreach ($module_data as $name => $data) {
-    if (!$data->status) {
-      drupal_classloader_register($name, dirname($data->filename));
-    }
+    drupal_classloader_register($name, dirname($data->filename));
   }
 }
 
+function simpletest_prepare_module_data() {
+  _system_rebuild_module_data('simpletest_module_data', TRUE);
+}
+
 /**
  * Generate test file.
  */
diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2636,21 +2636,19 @@
  * @return
  *   An associative array of module information.
  */
-function _system_rebuild_module_data() {
+function _system_rebuild_module_data($state_name = FALSE, $save = FALSE) {
   static $module_store = array(), $profiles_seen;
 
   $profile = drupal_get_profile();
   // Find modules not yet processed.
   $modules = array();
-  // If this is the first time, start with all modules.
-  if (empty($module_store)) {
+  if (empty($module_store) && ($state_name || drupal_valid_test_ua()) && ($data = state()->get($state_name ?: __FUNCTION__))) {
     // During a test run no modules can move, no info files can change so
     // speed up test runs by only finding and parsing once.
-    if (drupal_valid_test_ua() && ($data = state()->get(__FUNCTION__)))  {
-      $module_store = $data['module_store'];
-      $profiles_seen = $data['profiles_seen'];
-    }
+    $module_store = $data['module_store'];
+    $profiles_seen = $data['profiles_seen'];
   }
+  // If this is the first time, start with all modules.
   if (empty($module_store)) {
     // Find modules and installation profiles. As these are files, they can't
     // change within a request. However, it does depend on $profile.
@@ -2716,12 +2714,12 @@
     }
     // Store the processed modules.
     $module_store += $modules;
-    if (drupal_valid_test_ua()) {
-      state()->set(__FUNCTION__, array(
-        'module_store' => $module_store,
-        'profiles_seen' => $profiles_seen,
-      ));
-    }
+  }
+  if ($save || ($modules && drupal_valid_test_ua())) {
+    state()->set($state_name ?: __FUNCTION__, array(
+      'module_store' => $module_store,
+      'profiles_seen' => $profiles_seen,
+    ));
   }
   $modules = $module_store;
   // Do not return the non-active profile modules. Typically at most one: when
only in patch2:
unchanged:
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -68,6 +68,9 @@ drupal_set_time_limit(0);
 
 simpletest_script_reporter_init();
 
+// Prime module data.
+simpletest_prepare_module_data();
+
 // Execute tests.
 simpletest_script_execute_batch(simpletest_script_get_test_list());