Errors when running `pytest` with multiple processes

[ddundo]

It would be nice to run tests faster in the CI by using multiple processes (i.e., pytest -n auto). At the moment this raises these errors:

FAILED test/test_adapt.py::test_no_adapt[mmg2d] - Exception: Metric file does not exist! Path: /home/firedrake/animate/animate/.checkpoints/animate-checkpointth_zlgk6/adapted_mesh_checkpoint.h5.
FAILED test/test_checkpoint.py::TestCheckpointing::test_load - petsc4py.PETSc.Error: error code 76
FAILED test/test_adapt.py::test_no_adapt[mmg3d] - Exception: Adapted mesh file does not exist! Path: /home/firedrake/animate/animate/.checkpoints/animate-checkpointiutf64wq/adapted_mesh_checkpoint.h5.
FAILED test/test_utility.py::TestVTK::test_different_fnames - FileNotFoundError: [Errno 2] No such file or directory: '/home/firedrake/animate/test/tmp.pvd'
FAILED test/test_utility.py::TestVTK::test_different_lengths - FileNotFoundError: [Errno 2] No such file or directory: '/home/firedrake/animate/test/tmp.pvd'

It looks like we need to more robustly set these temporary checkpoint directories.