Functional tests purge cache before running. (develop)

[Fyorl]

http://tracker.phpbb.com/browse/PHPBB3-11190

[igorw]

I'm assuming this requires #1069 to be merged. It will not work, because the cache classes are different in 3.1. You need to create the cache driver instead of the cache class, and the includes will need to go away since those classes are autoloaded.

Can you make this PR include the commits from #1069 and then apply the changes I mentioned on top of them?

[Fyorl]

It doesn't require #1069 to be merged. In #1069 I write my own purge_cache method and get_cache_driver because they do not exist in the test framework in develop-olympus. In develop there is already a get_cache_driver and purge_cache function that uses the new cache and so I just call the already-available purge_cache here.

[igorw]

Missed that part, thanks for clarifying.