Add hacky parallel mpi test scheduler with subcommunicators

[msimberg]

This is an experiment (generated by an LLM). Testing to see if MPI tests run any faster with this in CI.

[msimberg]

cscs-ci run distributed

[msimberg]

cscs-ci run distributed

[github-actions]

Mandatory Tests

Please make sure you run these tests via comment before you merge!

• cscs-ci run default
• cscs-ci run distributed

Optional Tests

To run benchmarks you can use:

• cscs-ci run benchmark-bencher

To run tests and benchmarks with the DaCe backend you can use:

• cscs-ci run dace

To run test levels ignored by the default test suite (mostly simple datatest for static fields computations) you can use:

• cscs-ci run extra

For more detailed information please look at CI in the EXCLAIM universe.