test: assert UDP mux candidate port

[mertushka]

Purpose

Fix the post-merge Windows ARM CI failure in the UDP mux listener test.

Behavior Impact

• Test-only change.
• Keeps the UDP mux behavior assertion, but checks host candidate ports instead of requiring the SDP m-line port to be rewritten. Windows ARM can emit normal trickle SDP with m=application 9 while still advertising host candidates on the mux listener port.

Native / WPT Impact

• No runtime or native code changes.
• No WPT manifest changes.

Validation

• node --test test/nonstandard.test.js
• npm run check
• npm run wpt:selection:check
• git diff --check