test(skip): chmod does not restrict reads on Windows

[curdriceaurora]

Three tests across tests/integration/test_error_propagation.py and tests/plugins/test_base_coverage.py are skipped on Windows because chmod does not restrict read access there. Permanent Windows limitation. Related: #1027.

[curdriceaurora]

Closing — permanent platform guard or optional dep skip. Correctly skipped in CI, no action needed.