[ticket/12812] Add a migrator tool for config_text database changes

PHPBB3-12812
[ticket/12812] Improve migrator config_text tool tests

PHPBB3-12812
[ticket/12812] Improve migrator config_text tool tests

PHPBB3-12812
[ticket/12812] Remove unused global $db from tests

PHPBB3-12812
Merge pull request #2695 from VSEphpbb/ticket/12812

[ticket/12812] Add a migrator tool for config_text database changes

* VSEphpbb/ticket/12812:
  [ticket/12812] Remove unused global $db from tests
  [ticket/12812] Improve migrator config_text tool tests
  [ticket/12812] Improve migrator config_text tool tests
  [ticket/12812] Add a migrator tool for config_text database changes