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.

cache/tests/cache_test.php

@@ -63,10 +63,34 @@ public static function tearDownAfterClass() {
      * @return string
      */
     protected function get_expected_application_cache_store() {
+        global $CFG;
         $expected = 'cachestore_file';
+
+        // Verify if we are using any of the available ways to use a different application store within tests.
         if (defined('TEST_CACHE_USING_APPLICATION_STORE') && preg_match('#[a-zA-Z][a-zA-Z0-9_]*#', TEST_CACHE_USING_APPLICATION_STORE)) {
+            // 1st way. Using some of the testing servers.
             $expected = 'cachestore_'.(string)TEST_CACHE_USING_APPLICATION_STORE;
+
+        } else if (defined('TEST_CACHE_USING_ALT_CACHE_CONFIG_PATH') && TEST_CACHE_USING_ALT_CACHE_CONFIG_PATH && !empty($CFG->altcacheconfigpath)) {
+            // 2nd way. Using an alternative configuration.
+            $defaultstores = cache_helper::get_stores_suitable_for_mode_default();
+            $instance = cache_config::instance();
+            // Iterate over defined mode mappings until we get an application one not being the default.
+            foreach ($instance->get_mode_mappings() as $mapping) {
+                // If the store is not for application mode, ignore.
+                if ($mapping['mode'] !== cache_store::MODE_APPLICATION) {
+                    continue;
+                }
+                // If the store matches some default mapping store name, ignore.
+                if (array_key_exists($mapping['store'], $defaultstores) && !empty($defaultstores[$mapping['store']]['default'])) {
+                    continue;
+                }
+                // Arrived here, have found an application mode store not being the default mapped one (file),
+                // that's the one we are using in the configuration for sure.
+                $expected = 'cachestore_'.$mapping['store'];
+            }
         }
+
         return $expected;
     }