Test that allow_failure feature and allowed exit_codes functions as expected

We have a feature that allows specific types of failures to occur using the allow_failure and exit_codes keywords as explained https://docs.gitlab.com/ee/ci/yaml/#allow_failureexit_codes

We should add tests to validate that they work as expected.

This might be tricky to do because we don't want to fail our own pipeline but would want to test both allowed and non-allowed failures.