Add code to ApcCacheTestCase::setUp() to verify the cache bins in ApcCacheTestCase::$cacheBins are effectively handled by DrupalApcCache

ApcCacheTestCase::setUp() sets the persistent variables that change the cache back-end for the cache bins stored in ApcCacheTestCase::$cacheBins.
It should also verify the returned objects for those cache bins are instances of DrupalApcCache. If, for any reason, the returned objects are not instances of DrupalApcCache, the tests should not run, as in the better case we are testing the database cache back-end; in the worst case, we are testing an unknown class instances.