Remove wasteful double-caching of settings metadata

Remove wasteful double-caching of settings metadata

Settings metadata was being cached once per domain and one extra time for good measure.
hook_civicrm_alterSettingsMetaData() was invoked even when fetching cached data,
even though the hook-altered data was stored in the cache.

Now that the hook is invoked only once and the results cached, tests have been tweaked
to cache after setting the hook instead of before.