Opt-out of exempting tests

[untitaker]

When Homu sees a pull request with passing tests that can be fast-forwarded, it will skip running the tests, as shown in pimutils/vdirsyncer#391.

However: We skip some tests for pull requests, and only run them on the auto branch for performance reasons. The idea is that, like with the Rust project, the homu/bors bot is used for the "final" test run where even obscure/slow tests are executed.

It would be nice to have an option in the UI to disable exempting tests completely.