Skip tests that need symlink privileges instead of erroring

[Cyrus580529]

Summary

• Move the symlink capability probe out of test/test_index.py into test/lib/helper.py as symlinks_supported(), and add a requires_symlinks skip marker beside it.
• Apply the marker to the three tests that call os.symlink unguarded and error with OSError: [WinError 1314] on a Windows account without Developer Mode or SeCreateSymbolicLinkPrivilege.
• Drop the two existing copies of the check: the private probe in test_index.py, and the inline try/except plus skipTest in test_refs.py.

The tests that were erroring:

• test_repo.py::TestRepo::test_ignored_raises_error_w_symlink
• test_util.py::TestRmtree::test_avoids_changing_permissions_outside_tree
• test_installation.py::TestInstallation::test_installation

CI doesn't show this because the GitHub Windows runners hold the privilege, so the three run and pass there.

One behaviour note: the probe used to return early on non-Windows, so it never created anything. Now it always creates and removes a symlink in a temporary directory, on every platform.

Validation

Windows 11, Python 3.13, after ./init-tests-after-clone.sh:

• python -m pytest --ignore=test/performance — before: 6 failed, 711 passed, 47 skipped. After: 3 failed, 711 passed, 50 skipped.
• The three remaining failures are unrelated to symlinks: test_commit_msg_hook_fail, test_pre_commit_hook_fail, and test_index_mutation, which passes a POSIX absolute path to git rm.
• python -m ruff check test/
• python -m ruff format --check test/