Add outer timeout for mpi testing

[cpjordan]

Description

Fixes thetisproject/thetis#459 as demonstrated on thetisproject/thetis#438 (@connorjward).

This PR adds a shell-level timeout around each split job run by firedrake-run-split-tests.

The existing pytest timeout catches long-running pytest items, but it does not protect the whole mpiexec ... pytest process tree. We have observed (in Thetis - locally and in CI) MPI split jobs that print pytest success and then hang indefinitely during PETSc/OpenMPI finalization. In that state, the wrapper never writes jobN.errcode, GNU parallel never returns, and CI remains stuck.

Change

Each split job now runs as:

timeout --kill-after=${kill_after} ${outer_timeout} ${pytest_cmd}

inside the existing tee pipeline.

The timeout settings are configurable:

FIREDRAKE_RUN_SPLIT_TESTS_TIMEOUT=1800s
FIREDRAKE_RUN_SPLIT_TESTS_KILL_AFTER=60s

This PR assumes GNU timeout is available, matching the current Linux CI environment where firedrake-run-split-tests is used. If this helper is later used in macOS CI, a follow-up can either install Homebrew coreutils and use gtimeout, or add a small platform-specific wrapper.

[connorjward]

This should probably go into release so it becomes available for Thetis sooner.

[connorjward]

In your other PR you allow for GNU parallel not being there. This should also be optional no?

[connorjward]

And we definitely want this to run on macOS, so making it optional would help with that if you don't want to setup gtimeout as you suggested.

[cpjordan]

That makes sense but I was waiting for #5147 to be merged (I don't have the permissions to do so).

Do you want me to make the target branch for #5147 to be release as well? Then I can update this branch for your comments.

[connorjward]

Yeah #5147 should also go into release. I plan to merge that ASAP

[cpjordan]

I've updated the base on #5147, will update this branch once merged