Improve performance of common subexpression elimination and parallel testing

[rihi]

Currently common subexpression elimination uses lists to store which instructions use a specific expression. This PR replaces this usage with pythons Counter class to improve asymptotic runtime.

Additionally this pr adds parallelization to tests using pytest-xdist.

[blattm]

The first part of the PR (common subexpression elimination) is looking very good! I just fixed the typing issue you encountered add added some further notes. I think this part is ready to be be merged.

The second part (parallel tests) also seems to be working just fine. However, I am a little concerned about the limitations [1] concerning a consistent order of test parameters. Random errors thrown by pytest-xdist (or worse: maybe even missed tests) should be avoided.
Can we guarantee a consistent order? I already verified, that we only use lists in @pytest.mark.parametrize. It also seems like all functions called to generate such lists produce their output in a well-defined order. However conftest.py uses e.g. glob, which uses an arbitrary order. Applying a sort function seems like a reasonable Idea.

• Please ensure that all tests from conftest.py will be in a well-defined order. Please include comments in your work about the necessity of well-defined orders because of parallel testing.

• Do you come to the same conclusion about the "normal" test parameter lists all having a well-defined order?

Thank you for your contribution so far!

[1] https://pytest-xdist.readthedocs.io/en/stable/known-limitations.html

[rihi]

I didn't know about that limitation of pytest-xdist. That is a little annoying.
I've changed the code in conftests.py so that test cases are sorted alphabetically by their id (sample_name::function_name), which should provide a deterministic ordering. I've also checked all usages of pytest.mark.parametrize and also think that they all should be deterministic.

[blattm]

Thank you! I will wait for the current run of the extended pipeline to finish and then merge if it all went well.