PIPE-3027: add `@pytest.mark.mpi` marker and `--ignore-mpi-markers` option
- Add `mpi` pytest marker to `pyproject.toml`
- Auto-skip `@pytest.mark.mpi` tests in pytest_collection_modifyitems
when `MPIEnvironment.is_mpi_enabled` is False
- Add `--ignore-mpi-markers` CLI flag to disable the auto-skip, allowing
mpi-marked tests to run in a serial session
- Apply `@pytest.mark.mpi` to tests that require an MPI-enabled CASA
session: two alma_if fast tests and one vla fast test
