Test with latest Bundler

[deivid-rodriguez]

I checked if it passes all tests.

There was an issue, but I fixed it upstream though rubygems/rubygems#7066.

[deivid-rodriguez]

I figured out the issue.

Current tests use plugin as a git source pointing to a branch. However, I think git rev-parse --abbrev-ref HEAD is returning HEAD in CI. Old Bundler dealed with that fine because it cloned full repositories but Bundler 2.4 only clones the revisions needed and HEAD is ambiguous.

I think this is a test-only issue and I changed the tests to use a path source instead to fix that. I think path is more adequate anyways because it allows to test your changes more directly.

[jenshenny]

Thanks!