[ticket/12561] Add "after" check to schema_generator for columns_add

PHPBB3-12561
[ticket/12561] Added test for "after"

PHPBB3-12561
[ticket/12561] Add check to see if "after" column actually exists

If not, just append to the end

PHPBB3-12561
[ticket/12561] Added tests for "after last", "after missing" and "empty"

Also removed tabs

PHPBB3-12561
[ticket/12561] Reworked tests by nickvergessen

PHPBB3-12561
Merge pull request #2474 from PayBas/ticket/12561

[ticket/12561] Add "after" check to schema_generator for columns_add

* PayBas/ticket/12561:
  [ticket/12561] Reworked tests by nickvergessen
  [ticket/12561] Added tests for "after last", "after missing" and "empty"
  [ticket/12561] Add check to see if "after" column actually exists
  [ticket/12561] Added test for "after"
  [ticket/12561] Add "after" check to schema_generator for columns_add