test.support.os_helper.rmtree() does not clear BSD file flags

[serhiy-storchaka]

On BSD systems a test may leave behind files or directories with flags such as UF_IMMUTABLE or UF_NOUNLINK set, which prevent the directory from being modified or removed. os_helper.rmtree() retries with os.chmod() but never clears the flags with os.chflags(), so it fails to remove such content.

This surfaced as a regrtest WORKER BUG on DragonFly: test_tempfile left UF_NOUNLINK-flagged directories in the worker's temporary directory (gh-154307), and the worker's os_helper.rmtree() cleanup then crashed with PermissionError, killing the workers and aborting the run.

os_helper.rmtree() should clear file flags before removing.

Linked PRs

• [3.15] gh-154308: Clear file flags in os_helper.rmtree() (GH-154310) #154314
• [3.14] gh-154308: Clear file flags in os_helper.rmtree() (GH-154310) #154315
• [3.13] gh-154308: Clear file flags in os_helper.rmtree() (GH-154310) #154316