Parametrization-based test scheduling

I have a test matrix in which the same test suite is run across multiple environments. It's implemented using a fixture which is parametrized with the set of environments, and all tests use it.

I'd like to use xdist, but I couldn't find a test load scheduler that'd fit: All tests running on the same environment must execute sequentially (no specific order is required, of course, but they mustn't run concurrently).

After a quick look under xdist/schedulers, I patched xdist.schedulers.loadfile.LoadFileScheduling._split_scope to split based on the value of the parametrized fixture (which, luckily, appears in the nodeid). This works just fine, but now I'd like to do it without patching xdist in runtime :)

So first of all, if there's any existing way to schedule parametrized tests, I'd be happy to use it instead... And if not, I could use some design help on how my patchy scheduling can be written so it's incorporated into xdist (I'd be happy to create a PR, I think it's quite useful, it was also requested in the past, e.g #324).

I was thinking a --dist=param/nodeid_regex/ argument, then it'd split based on the result.

Start in xdist/schedulers/loadfile.py, especially LoadFileScheduling._split_scope, and review the existing scheduler behavior and the parametrized fixture pattern described in the issue. Determine how scheduling should group tests by parametrized environment and define the behavior that ensures tests sharing an environment never run concurrently.