Add unit test for LaplaceXZPetsc

[dschwoerer]

No description provided.

[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]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: no header providing "bout::globals::mesh" is directly included [misc-include-cleaner]

tests/integrated/test-laplacexz/test-laplacexz.cxx:10:

- #include <bout/bout.hxx>
+ #include "bout/globals.hxx"
+ #include <bout/bout.hxx>

[github-actions]

warning: using decl 'mesh' is unused [misc-unused-using-decls]

using bout::globals::mesh;
                     ^

Additional context

tests/integrated/test-laplacexz/test-laplacexz.cxx:16: remove the using

using bout::globals::mesh;
                     ^