Tests

Refactor test_petsc_laplace for pytest #1055

Job	Run time
Shared, OpenMP, 3D metrics	24s
Fedora	25s
PETSc unreleased, ADIOS2, 3D metrics	24s
Default options, Ubuntu 22.04	23s
Optimised, static	24s
Debug, shared	25s
Shared, release	25s
	2m 50s

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