MDL-55610 behat: Clear static file cache

php file cache might be out-of-sync
to ensure full directory is deleted
clear php file cache

lib/moodlelib.php

@@ -1621,6 +1621,7 @@ function purge_all_caches() {
     cache_helper::purge_all();
 
     // Purge all other caches: rss, simplepie, etc.
+    clearstatcache();
     remove_dir($CFG->cachedir.'', true);
 
     // Make sure cache dir is writable, throws exception if not.