[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