[ticket/10839] Always run functional tests by default

They are skipped anyway, if $phpbb_functional_url is not configured.
If you want to run tests without functional tests, just append
    --exclude-group functional
to your call

PHPBB3-10839
[ticket/10839] Removing phpunit.xml.all

To run all tests, just append
    --group slow
to your call

PHPBB3-10839
[ticket/10839] Fix note about running slow tests and build task

PHPBB3-10839