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

We need to rename the index key because with the default prefix (phpbb_)
this key has a length of 31. And because we don't accept the keys longer
than 30 characters we should ensure that by default no key is longer than
30 characters.

PHPBB3-12873
[ticket/12873] Don not touch the existing migrations

PHPBB3-12873
[ticket/12873] Add migration to rename the index

PHPBB3-12873
Merge pull request #2761 from Nicofuma/ticket/12873

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

* Nicofuma/ticket/12873:
  [ticket/12873] Add migration to rename the index
  [ticket/12873] Don not touch the existing migrations
  [ticket/12873] Test the good identifier in \phpbb\db\tools