[AUDIO_WORKLET] Fix spinlocks for audio worklets (and add tests)

[cwoffenden]

Fix for #22962. In brief:

• spinlocks fixed so they work in audio worklets
• adds tests for various spinlock cases
• test renamed to match its new use
• test converted to C-only (like the other tests, plus it wasn't using C++ features)

This is an update to #22995 which had become hard to track. The spinlocks are moved to using emscripten_get_now() so they can be used in Audio Worklets, and a set of interactive tests added for the spinlock use cases.

The test can be run with:

test/runner interactive.test_audio_worklet_emscripten_locks

Or standalone with:

emcc -sAUDIO_WORKLET -sWASM_WORKERS -pthread audioworklet_emscripten_locks.c

I can change the other emscripten_performance_now() calls to emscripten_get_now() (as suggested in #22995) in a separate PR.

[cwoffenden]

Odd, GH closed this when I renamed the branch to merge into a fork (it's residing in cw-audio-spinlock-fix).

If there's any interest in this I can re-open another PR.