MDL-53566 core: Add support for context locking

This chagne adds support for a new feature known as Context Locking.
This allows a context to be locked, thereby removing all write
capabilities for all users (including admin) for that context, and all
child contexts.

lib/phpunit/classes/advanced_testcase.php

@@ -132,6 +132,9 @@ abstract class advanced_testcase extends base_testcase {
             self::resetAllData(true);
         }
 
+        // Reset context cache.
+        context_helper::reset_caches();
+
         // make sure test did not forget to close transaction
         if ($DB->is_transaction_started()) {
             self::resetAllData();