CI: add mpi/distributed module to free CI pipe

[iarejula-bsc]

Adds three free CI jobs, all using minidmr, an open-source tool built by our team that allows creating a reproducible HPC cluster on Docker to simulate a multi-node MPI cluster . minidmr is already integrated into the CI pipelines of two of our other projects, and is also used to run MPI tests locally.

By using it on QuEST, this lets your CI pipeline run MPI tests on a single free ubuntu-latest runner. test_paid.yml is untouched for now; however, after a careful review, it could be removed in a follow-up to save money.

This PR adds:

1. test_free.yml: distributed-minidmr-test: full functional tests over real MPI (minidmr, 4 simulated nodes).
2. audit.yml: sanitisation-test-mpi: ASan over comm/* (the MPI code), minidmr with 2 nodes : the first time this code has run under a sanitiser.
3. audit.yml: coverage-test-mpi: lcov over comm/* via minidmr, 2 nodes: complements the existing serial coverage job, which can't exercise MPI.

Impact:

• Cost: replaces the paid equivalent (mpi=ON, cuda=OFF in test_paid.yml, ~$4.80/run, only triggerable manually via workflow_dispatch) with free jobs on every push/PR, saving ~$5/run.
• Coverage: comm/* goes from 0% measured (explicitly excluded from the serial coverage job, which doesn't compile with MPI) to comm/comm_config.cpp at 77.4% and comm/comm_routines.cpp at 4.3% line coverage.

[iarejula-bsc]

added author info in the header per convention, not sure if this is correct, might be better placed in the headers of the new test instead. Happy to change if needed.

[iarejula-bsc]

i run the tests on mare nostrum 5 and on minidmr local and gh actions, same results on all of them

[otbrown]

Hi @iarejula-bsc, thanks for this!

We're currently trying to (finally) get the 4.3 release ready, as well as buried under proposal writing, so will probably park this at least until 4.3 is out, but I like it!

Note that I currently don't expect the tests to pass when run on multiple processes, but that is one of the items on my list for post-4.3. Your contribution will certainly help with working towards that.

[iarejula-bsc]

We're currently trying to (finally) get the 4.3 release ready, as well as buried under proposal writing, so will probably park this at least until 4.3 is out, but I like it!

Glad to hear that!

Your contribution will certainly help with working towards that.

I woudl like to say that minidmr can we also used in the local machine as a local shell to compile/run/test/debug code, if you feel this will help u to develop faster the MPI code dont hestiate on pinging me and will try to explain how to use it :D