TST: Use xfail instead of skip to mark expected failures

[keileg]

The (deprecated) coarsening module has a few tests that are expected to fail. These used to be marked with @pytest.mark.skip, but this is confusing, furthermore, the test make the weekly test run (which includes skipped tests) fail. Using xfail instead should resolve this.

Proposed changes

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Types of changes

What types of changes does this PR introduce to PorePy? Put an x in the boxes that apply.

• [ ] Minor change (e.g., dependency bumps, broken links).
• [ ] Bugfix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] Testing (contribution related to testing of existing or new functionality).
• [ ] Documentation (contribution related to adding, improving, or fixing documentation).
• [ ] Maintenance (e.g., improve logic and performance, remove obsolete code).
• [ ] Other:

Checklist

Put an x in the boxes that apply or explain briefly why the box is not relevant.

• [x] The documentation is up-to-date.
• [x] Static typing is included in the update.
• [x] This PR does not duplicate existing functionality.
• [x] The update is covered by the test suite (including tests added in the PR).
• [x] If new skipped tests have been introduced in this PR, pytest was run with the --run-skipped flag.