Do not use Makefile for firedrake-check

[connorjward]

• The previous approach led to issues for a number of users for whom 'mpiexec' did not work inside the Makefile.
• We now support a '--mpiexec' argument for those requiring extra commands like '--oversubscribe' or 'srun'.
• Adapts to mpi-pytest change so parallel[1] covers all serial tests.

This is approved an can be merged once a new release of mpi-pytest is made. (now done)

[JHopeCollins]

I've suggested refactoring how the tests to run are specified so it'd be easier to add other parallel tests with nprocs!=3 in the future. What do you think?

[connorjward]

@JHopeCollins I have followed your suggestions though the code doesn't exactly match your suggested changes.

[JHopeCollins]

Very happy with this, thanks @connorjward!

[connorjward]

This is failing because the non-forking mpiexec problem is alive and kicking! I have a fix in this instance but it's interesting that this works fine on my machine.

[connorjward]

Merging as this was previously approved and passes CI in the relevant places.