Improve defaults for testing suite

[hyperupcall]

I started to test @vanpipy's changed on my machines. I think this changes will streamling the testing process for other people as well.

I also wish to close the following issues:

Closes #1083 (spam)
Closes #818 (question answered)
Closes #486 (out of scope)

[hyperupcall]

@spacewander this should be ready - The only change to CI was using checkout@v4 (which still works) and updated the git.Repo.init(repo_work_dir, b="default") call to manually set the default branch. I set it to default, which is different than main and master to make things a bit less confusing (in my opinion)