[12.x] Test configurationIsCached

[seriquynh]

The application configurationIsCached method is currently controlled by 2 factors:

• The "config_loaded_from_cache" abstract is bound to the container or not.
• A config cache file (usually bootstrap/cache/config.php) exists or not.

Therefore, I added some small tests here to prevent unexpected breaking changes later if any.

a config cache file does not exist

• If "config_loaded_from_cache" is not bound to the container, configurationIsCached returns false and "config_loaded_from_cache" is implicitly set as false
• If "config_loaded_from_cache" is bound to the container as true, configurationIsCached returns true.
• If "config_loaded_from_cache" is bound to the container as false, configurationIsCached returns false.

a config cache file exist

• If "config_loaded_from_cache" is not bound to the container, configurationIsCached returns true and "config_loaded_from_cache" is implicitly set as true
• If "config_loaded_from_cache" is bound to the container as false, configurationIsCached returns false.

Plus

inside the RegisterProviders class, instead of accessing config_loaded_from_cache directly, using configurationIsCached is more consistent because:

• config_loaded_from_cache should be called with a "setter" in order to change the config cache state.

$app->instance('config_loaded_from_cache', true);

• configurationIsCached should be called with a "getter" in order to check the config cache state.

$app->configurationIsCached();

[taylorotwell]

If nothing is broken here I don't want to change the logic in the bootstrapper on a patch release.