Should detect MPI library and fail gracefully when forking is not possible

[connorjward]

Currently running parallel tests in 'forking' mode (i.e. mpiexec on the inside) gives a cryptic error for non-MPICH MPI:

E               subprocess.CalledProcessError: Command '['mpiexec', '-n', '1', '-genv', '_PYTEST_MPI_CHILD_PROCESS', '1', '/home/sa/M4R/ven4r/bin/pytest', '--runxfail', '-s', '-q', '/home/sa/firedrake/tests/firedrake/vertexonly/test_vertex_only_mesh_generation.py::test_generate_cell_midpoints_parallel[interval-redundant]', ':', '-n', '2', '/home/sa/M4R/ven4r/bin/pytest', '--runxfail', '-s', '-q', '/home/sa/firedrake/tests/firedrake/vertexonly/test_vertex_only_mesh_generation.py::test_generate_cell_midpoints_parallel[interval-redundant]', '--tb=no', '--no-summary', '--no-header', '--disable-warnings', '--show-capture=no']' returned non-zero exit status 1.

/usr/lib/python3.10/subprocess.py:526: CalledProcessError
---------------------------------------------------------------------------------------------- Captured stderr call -----------------------------------------------------------------------------------------------
mpiexec: Error: unknown option "-genv"
Type 'mpiexec --help' for usage.

We really want to have something like

Your MPI implementation does not support running mpi-pytest in 'forking' mode. Use mpiexec -n # ... -m parallel[#] instead...

[connorjward]

Closed by #21