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GH-43076: [C#] Upgrade Xunit and change how Python integration tests are skipped (#43091)

### Rationale for this change

See #43076. The previous Xunit upgrade was reverted due to this breaking how the Python C Data Interface integration tests were skipped. It looks like this is unlikely to be fixed in xunit or xunit.skippablefact soon (see https://github.com/AArnott/Xunit.SkippableFact/issues/32), so I've refactored the tests to work around the issue.

### What changes are included in this PR?

Re-update xunit to 2.8.1 and refactor the `CDataSchemaPythonTest` class construction so that skipping these tests when the `PYTHONNET_PYDLL` environment variable isn't set works again.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #43076

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>