The current APCu cache driver implements a global clearing of the APCu
when the phpBB cache is purged. This is inappropriate if there are other
phpBB boards, or other php applications sharing the APCu cache.
This patch changes the behviour so that only cache entries matching
the key_prefix of this board are cleared by a phpBB cache purge.
The APCu unit test script has been updated to test this behaviour.
It has also been updated so that the test case can be run individually
previously it relied on initialisations made in other test scripts.
PHPBB3-15726
Add new cache driver apcu.php (based closely on existing APC cache driver)
Add new unit test apcu_driver_test.php for the new driver
Update RUNNING_TESTS.md to clarify requirements for apc, apcu and apc_bc extensions
and add a couple of general hints to RUNNING_TESTS.md
PHPBB3-15247
