Test more thoroughly

[zofrex]

The testing workflow (.github/workflows/testing.yml) and testing repo work in concert to test that this action does actually mirror a branch successfully. However, they only test a basic scenario (the branch can be fast-forwarded) with the default settings.

These additional scenarios, and possibly others, should also be tested:

• Default settings, non-fast-forward – should work
• Force push disabled, fast-forward – should work
• Force push disabled, non-fast-forward – should not work

Most of the work here is already done for the existing test, but it needs to be duplicated and altered for the above scenarios.