Treat background migrations as backend code so we run all background migration tests when there are backend changes

This is proposed at gitlab-org/quality/engineering-productivity/master-broken-incidents#491 (comment 1221913616) and a related issue is: #386728 (closed)

Given the fact that background migrations can use application backend code, changing application backend code can break background migrations. Given this, we should also run background migration tests when there are backend changes.

To show the idea of this, it'll look like this:

Edited: The suggested diff is quite wrong and can be misleading, thus I removed it. I'll fill it here once I have something to show the idea.

There are more patterns we need to apply the same idea, and we might want split background migration from rspec migration pg12 so it's more clear what they're running. Maybe rspec background-migration pg12.