This is the first step of making the test suite use a single directory
for writing files to, this directory being tests/tmp.
The cache test, instead of tests/cache/tmp, now writes to tests/tmp/cache.
Also remove cache directory in setUp method, in case an earlier test run
did not complete successfully and the cache directory was not cleaned up.
Finally, this change makes the cache test take responsibility over the entire
contents of its cache directory.
PHPBB3-10013
They are now refered to as cache drivers rather than ACM classes. The
additional utility functions from the original cache class have been
moved to the cache_service. The class loader is now instantiated without
a cache instance and passed one as soon as it is constructed to allow
autoloading the cache classes.
PHPBB3-9983
