[ticket/13232] Use a cache var with _ to force TTL and shuffle cron tasks

PHPBB3-13232
[ticket/13232] Fix indentation

PHPBB3-13232
[ticket/13232] Fix more issues with TTL and global cache file

PHPBB3-13232
Merge pull request #4 from nickvergessen/naderman-ticket/13232

[ticket/13232] Fix more issues with TTL and global cache file
[ticket/13232] Ensure prune cron job always gets run in prune test

The cron scheduler might pick a different task but we don't want to go
through them all to find the one we need. So we just make sure cron is
run at all and then manually run the one we want.

PHPBB3-13232
[ticket/13232] Fix spaces vs tabs

PHPBB3-13232
Merge branch 'naderman-ticket/13232' into prep-release-3.1.0