[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
* exreaction/ticket/11881:
[ticket/11881] Move convert_timezones_test to migrator
[ticket/11881] Fix test filename
[ticket/11881] Limit to 500
[ticket/11881] Make sure user_timezone isn't converted twice
[ticket/11881] Better split the timezone conversion into chunks; add test
[ticket/11881] Timezone migration can take a long time