Skip symlink tests when the platform forbids creating one - #5390

Fixes #5389.

What

Three tests in TestFileCollection create symlinks without any guard, so the test suite fails with OSError: [WinError 1314] for any Windows contributor who is neither elevated nor running with Developer Mode enabled:

• test_get_sources_symlink_and_force_exclude
• test_get_sources_with_stdin_symlink_outside_root
• test_get_sources_with_stdin_filename_and_force_exclude_and_symlink

This is the same problem #287 fixed in 2018 for test_broken_symlink — that guard is still in place and working, but the three tests above were added later without it. CI cannot catch the regression because the Windows runner can create symlinks; only an ordinary local Windows checkout fails.

Change

Add a module-level symlink_or_skip(link, target) helper next to the other test utilities and use it at all five symlink_to call sites in TestFileCollection:

def symlink_or_skip(link: Path, target: Path | str) -> None:
    try:
        link.symlink_to(target)
    except (OSError, NotImplementedError) as e:
        pytest.skip(f"Can't create symlinks: {e}")

Notes:

• pytest.skip instead of self.skipTest, since TestFileCollection is a plain pytest class, not a unittest.TestCase (test_broken_symlink keeps its existing skipTest guard).
• Centralising the helper means the next symlink test picks up the guard by default, which is what would have prevented this recurrence.
• The fix follows the approach suggested in the issue (thanks @iam-kira for the detailed report and verification).

Verification

• Before: 3 failed, 442 passed on Windows (WinError 1314) — matches the issue report.
• After: 0 failed / 442 passed, with the three tests reporting SKIPPED (Can't create symlinks: ...).
• Full suite: 442 passed, 7 skipped, 8 subtests passed (pytest --ignore=tests/test_blackd.py).
• black --check tests/test_black.py, flake8, isort --check-only, and mypy all pass.

Changelog

Test-only change with no user-facing behaviour impact, so I believe no CHANGES.md entry is needed — happy to add one under the appropriate heading if you prefer.