MDL-48568 cache: make tests aware of default application store

When an alternative cache configuration is used, aka:

define('TEST_CACHE_USING_ALT_CACHE_CONFIG_PATH', true);
$CFG->altcacheconfigpath = '/tmp/xxxxx.php';

the get_expected_application_cache_store() method was not able of
retroffiting the default application store being used.

This commit just implements that functionality so, when
executing unit tests using alternative configuration, assertions
looking for that will pass without a problem.