[ticket/12873] Test the correct identifier in \phpbb\db\tools

https://tracker.phpbb.com/browse/PHPBB3-12873
https://tracker.phpbb.com/browse/PHPBB3-12770

PHPBB3-12770
PHPBB3-12873
Arg, this seems like a bad idea. It will cause 2 different index names depending on whether you install 3.1 or updated from 3.0
However dropping the index and recreating it might be even more expensive. But I guess it's what we should do.