chore: make capture tests portable

[abrichr]

Why

The larger Windows test matrix on main found two test assumptions after #126 merged. POSIX mode bits do not have the same meaning on Windows. A live monotonic-clock calculation can also cross an exact floating-point boundary. The product behavior passed; only these assertions failed in the main build.

What changed

• Keep the empty authentication-manifest content check on every platform. Check owner-only mode bits on POSIX.
• Give the macOS accessibility-timeout test a fixed monotonic clock.

This leaves capture and authentication behavior unchanged.

Verification

• pytest for both affected tests: 2 passed
• ruff check --no-cache for both affected files
• git diff --check