MDL-52656 unittest: Reset original session while setting user

lib/phpunit/classes/advanced_testcase.php

@@ -492,6 +492,9 @@ abstract class advanced_testcase extends base_testcase {
         unset($user->access);
         unset($user->preference);
 
+        // Enusre session is empty, as it may contain caches and user specific info.
+        \core\session\manager::init_empty_session();
+
         \core\session\manager::set_user($user);
     }