dbmate check for testing migrations can be rolled back?

[fragglet]

Hi, as mentioned in the top-level README:

By default, dbmate doesn't know how to roll back a migration. In development, it's often useful to be able to revert your database to a previous state.

Rollbacks are obviously incredibly important and it would be nice to have the confidence that a migration not only has a migrate:down step, but that it definitely functions as an inverse of the migrate:up step - even if one has been implemented, you don't know it works until it has really been tested.

It would be nice if there was something like a dbmate check or dbmate test command. Such a command would create an empty (eg. in-memory) database, automatically run through all migrations while saving the schema at each step, then automatically run through all rollbacks and check the schema matches at each stage, printing a detailed error message if any discrepancy is found. Then a user can add the check to their CI system.

Apologies if this is not a new idea - I couldn't find anything like it described in the documentation, and I also checked past feature requests. I'd potentially be willing to implement such a feature if you think it's a good idea.