Use only a test method for ApcCacheSaveCase

The ApcCacheSaveCase class has a test method for each PHP data type which could be cached. A single test method is sufficient.

The other test method verifies there are no empty cache IDs when user/register is accessed.That test can be removed, since it does not test the cache back-end, but what the page callback for the user/register route does, and Drupal core tests already check that.