Alembic migrations should have associated test cases

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

Migrations can be very error prone, and unusual or unexpected state on a prod server can cause migrations to fail. We can try to anticipate these and write tests cases to ensure migrations are always successful.

Describe the solution you'd like

SecureDrop has a strategy in test_alembic.py and the related tests/migrations directory.

Describe alternatives you've considered

I do not know of any other ways people test Alembic migrations. More research might lead us to a better solution.

Additional context

I have seen so many "lgtm" migrations fail in prod :(( the problem is real.