Add `travel_back` to remove stubs from `travel` and `travel_to` and remove auto-rollback after each test case

This is a alternative implementation of https://github.com/rails/rails/pull/12966.

The reason is the same plus, I'm removing the auto-rollback after each test case. The reason is this auto-rollback will only work with minitest out of the box and it may confuse the users.

Closes #12966