Remove and ignore the dummy app's test sqlite database, take two

Another take on https://github.com/rails/activejob/pull/105, which broke the tests.

Rails' `test_help` helper configures ActiveRecord to automatically maintain the test schema, creating the test db if necessary. We need to give it a little hint, though, by telling it where to find the dummy app's migrations. Otherwise it won't be able to find the latest migration version and will assume there's nothing to do.