migration: Add connection utils for tests

Created by: efritz

Add NewTestDB that can be used for tests. This does not require the internal/database/migration/store package, which means that we will be free of import cycles when trying to test the migration packages themselves.

This implementation stores the migration state in memory rather in a table to avoid the cyclic dependency. The integration testing we have today is more than sufficient to catch the errors that could occur because we don't run our unit tests with the same store factory.