[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
@Nicofuma we decided to go with a new migration and just make the upgrade take a while.
So drop the old index and create a new one in a new migration.