Generate VOL tests for h5diff

[mattjala]

Depends on both #5551 and #5675. Marked as draft and won't pass CI until those are merged.

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Important

Add VOL tests for h5diff and update GitHub Actions to exclude H5DIFF tests from certain runs.

• Behavior:
  • Exclude H5DIFF tests from ctest runs in .github/workflows/vol_rest.yml.
  • Add VOL tests for h5diff in CMakeTests.cmake, generating test files for each external VOL connector.
• Testing:
  • Add ADD_SH5_TEST and ADD_PH5_TEST macros in CMakeTests.cmake to handle serial and parallel tests for VOL connectors.
  • Implement ADD_H5_UD_TEST macro for plugin tests in CMakeTests.cmake.
• Misc:
  • Create directories and copy necessary test files for VOL tests in CMakeTests.cmake.

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[byrnHDF]

I wonder if this should be a global variable - it's likely used in multiple areas

[jhendersonHDF]

Minor naming issue but otherwise this looks good