The test from #5950 could be expanded to also include stat, but git_path_exists and git_path_isdir doesn't specifically check errno and have no way of returning invalid path rather than non-existing path so it doesn't seem like a good option here.

Yeah, Windows is going to be the tricky bit here for sure. Let me do some testing over there.

Would it be possible to add
mkdir -p "<path-to-where-we-run-clone-test-from>/git:/github.com/libgit2/TestGitRepository"
to one of the docker builds?
That should make current tests fail if clone prioritizes git:/github.com/libgit2/TestGitRepository over git://github.com/libgit2/TestGitRepository without bothering anyone testing on Windows.

An alternative to checking the url for "://" could be to split up transport_find_fn() so that it can be asked if it is a known url, local path or file:// style url, but I don't know if we really gain anything from that.

I figured that the way to test this is to add a "http:/"-folder in the current directory for the online cloning tests.
I'm very uncertain on the proper way to do this in the clar environment so if @ethomson could take a look at it it would be great.

We can't really test this on Windows systems, but then again, it will never occur on those either.

Restructured and added comments to make the flow clearer.
I think it should be good to go, if it is OK to use git_futils_mkdir_r in the test initialization that is.