Add more tests for supported CIs

[christophebedard]

Follow-up to #2

Currently, the supported CIs are tested by actually running dco-check (via a test) in PRs. That only tests a portion of the features: it doesn't test pushing to the default branch.

This would be a bit more complex to test, since we would need to emulate the CI environments (i.e. define the right env var, provide the right data, etc.), create some commits, and run dco-check.

A first step could be to do this with a simple git repo that we set up in a test.