Parallel tests

[ddundo]

Closes #97. See discussion there.

Opening this to discuss/debug more easily.

Ideally we'd mark tests with mpi-pytest's parallel marker: for example, a test marked with parallel(nprocs=[1, 2, 3]) would be run in serial, as well as with 2 and 3 processes in parallel. But passing multiple values to nprocs like this doesn't seem to work for unittest-based tests. So here I have a workaround, where I mark tests with a new parallel_dynamic marker, which then gets replaced by parallel(nprocs=N), where N is whatever is passed to mpiexec -n N. This works well, but it's probably better to be explicit, so it's just a temporary solution.

[ddundo]

@stephankramer @jwallwork23 here's what I mentioned on the meeting today. I commented out the parallel_dynamic marker from tests that hang.

Based on the discussion in firedrakeproject/mpi-pytest#12, I tried installing pytest-timeout and then running mpiexec -n 2 python3 -m coverage run --parallel-mode --source=movement -m pytest -v -m parallel[2] --timeout=60 --timeout-method=thread --session-timeout=1200 test but the same tests still continued to hang.