MDL-47608 cachestore_mongo: fixed unit test error

If you configured your site to run mongo unit tests and had
the mongo extension and backend installed but did not have the
mongo service running you would get a fatal error when running
the unit tests.
That problem has been fixed by checking the mongo test instance
is ready before returning it.

cache/stores/mongodb/lib.php

@@ -556,6 +556,9 @@ class cachestore_mongodb extends cache_store implements cache_is_configurable {
         }
 
         $store = new cachestore_mongodb('Test mongodb', $configuration);
+        if (!$store->is_ready()) {
+            return false;
+        }
         $store->initialise($definition);
 
         return $store;