Add unit test for LaplaceXZPetsc

[dschwoerer]

[ZedThree]

See #2207 for some discussion

[ZedThree]

Possible directions for investigation:

• convergence checks for the round-trip test here: if the issue is one of mismatched discretisations, the error should decrease as we increase the resolution
• analytic tests: this could be done in both uniform and spatially-dependent metric tensors by solving the forward operator with something like sympy or mathematica -- this would give us analytic tests for both forward and inverse operators

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"