This PR:

1. adds to the CI db:rollback and db:migrate for the last few migrations. So this validates that migrations can apply and can revert on all supported databases.

2. On sqlite only, it verifies the resulting schema is the same as the original. This validates that new migrations are compatible with the updated db/schema.rb.

3. It removes a usage of unicode characters in one test, because it failed on mysql in some scenario. The same fix is also in Improve tests for review_metrics #675 .