Functional tests fail for extensions

The removal of cache purging from functional tests has broken functional testing on extensions.

Because, if the cache has stale components from a previous test, the functional extension setup script thinks the extension is installed, so it does not actually install the extension, thus all functional tests on the extension will fail.

So we need a cache purge before we setup extensions for functional testing.

Caused by https://tracker.phpbb.com/browse/PHPBB3-14394