test: refactor default P2D mesh setup into pytest fixture

[swastim01]

This PR introduces a pytest fixture for constructing the default P2D mesh and
uses it in a small number of finite volume unit tests to reduce repeated setup.

No test behaviour is changed.

Contributes to #4502

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[agriyakhetarpal]

Hi @swastim01, looks like methods of the name get_p2d_mesh_for_testing are defined in a few more places:

1. tests/integration/test_spatial_methods/test_spectral_volume.py
2. tests/unit/test_spatial_methods/test_spectral_volume.py
3. in tests/shared.py – used twice in tests/integration/test_spatial_methods/test_finite_volume.py (this was probably missed)

I'm happy to approve this once these references are handled!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (86531fb) to head (43fbdf4).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5376   +/-   ##
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  Coverage   98.29%   98.29%           
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  Files         328      328           
  Lines       28941    28941           
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  Hits        28449    28449           
  Misses        492      492           

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