diff -u b/core/includes/module.inc b/core/includes/module.inc
--- b/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -15,25 +15,16 @@
  *   Whether to load only the reduced set of modules loaded in "bootstrap mode"
  *   for cached pages. See bootstrap.inc. Pass NULL to only check the current
  *   status without loading of modules.
- * @param bool $reset
- *   (optional) Internal use only. Whether to reset the internal statically
- *   cached flag of whether modules have been loaded. If TRUE, all modules are
- *   (re)loaded in the same call. Used by the testing framework to override and
- *   persist a limited module list for the duration of a unit test (in which no
- *   module system exists).
  *
  * @return bool
  *   A Boolean indicating whether all modules have been loaded. This means all
  *   modules; the load status of bootstrap modules cannot be checked.
  */
-function module_load_all($bootstrap = FALSE, $reset = FALSE) {
+function module_load_all($bootstrap = FALSE) {
   static $has_run = FALSE;
 
-  if ($reset) {
-    $has_run = !$bootstrap;
-  }
   // Unless $boostrap is NULL, load the requested set of modules.
-  elseif (isset($bootstrap) && !$has_run) {
+  if (isset($bootstrap) && !$has_run) {
     $type = $bootstrap ? 'bootstrap' : 'module_enabled';
     foreach (module_list($type) as $module) {
       drupal_load('module', $module);
@@ -165,6 +156,9 @@
       $lists = $cached->data;
     }
     else {
+      if (_system_list_rebuild(TRUE) && $type == 'module_enabled') {
+        return drupal_map_assoc(array_keys(config('system.module')->get()));
+      }
       $lists = _system_list_rebuild();
       $lists['bootstrap'] = array_keys($lists['bootstrap']);
     }
@@ -179,7 +173,13 @@
 /**
  * Rebuild the system list data structure.
  */
-function _system_list_rebuild() {
+function _system_list_rebuild($test_only = FALSE) {
+  static $in_rebuild = FALSE;
+
+  if ($test_only) {
+    return $in_rebuild;
+  }
+  $in_rebuild = TRUE;
   // The module name (rather than the filename) is used as the fallback
   // weighting in order to guarantee consistent behavior across different
   // Drupal installations, which might have modules installed in different
@@ -237,7 +237,7 @@
     $lists['theme'][$key]->prefix = ($lists['theme'][$key]->info['engine'] == 'theme') ? $base_key : $lists['theme'][$key]->info['engine'];
   }
 
- foreach ($enabled_modules as $name => $weight) {
+  foreach ($enabled_modules as $name => $weight) {
     // Build a list of all enabled modules.
     $lists['module_enabled'][$name] = $name;
     $filename = $module_data[$name]->filename;
@@ -256,6 +256,7 @@
   }
   cache('bootstrap')->set('system_list', $lists);
   cache('bootstrap')->set('bootstrap_modules', $lists['bootstrap']);
+  $in_rebuild = FALSE;
   return $lists;
 }
 
diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -829,7 +829,6 @@
 
     // Reset module list and module load status.
     module_list_reset();
-    module_load_all(FALSE, TRUE);
 
     // Restore original in-memory configuration.
     $conf = $this->originalConf;
diff -u b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
--- b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Upgrade/UpgradePathTestBase.php
@@ -243,7 +243,6 @@
     // but not on the test client.
     system_list_reset();
     module_implements_reset();
-    module_load_all(FALSE, TRUE);
 
     // Rebuild caches.
     drupal_flush_all_caches();