test: check symlink refusal by errno instead of message

[kudala-bharani]

Fixes #731.

The lock is already rejected correctly on musl, but the test fails because musl says Symbolic link loop and the regex expects different wording. Check errno instead, accepting ELOOP and the existing NetBSD EFTYPE case. The real filesystem test and its capability checks stay in place; library behavior is unchanged.

The reported wording fails before this change and passes after it in a local simulation. I also checked different error text, platforms without EFTYPE, and an unrelated EACCES error that must still fail. These are simulations on macOS, not an actual musl run.

Validation on macOS / Python 3.11.15:

• tox run -e 3.11: 1,457 passed, 53 skipped; 100% line, branch, and diff coverage.
• Type checking for the changed test module and the Towncrier check pass.
• Repository hooks pass locally except yamlfmt, whose Go dependency download failed. No YAML files changed, and the full GitHub pre-commit check passes.

Includes the required changelog fragment. GitHub's documentation build passes; the remaining Actions workflows need maintainer approval.