[AUDIO_WORKLET] Rewrite lock test to be more realistic

[cwoffenden]

This is a complete rethink of the lock test to do something locks would actually be used for: maintain atomicity of a complex data structure. We have this dummy struct with three members:

typedef struct {
  uint32_t val0;
  uint32_t val1;
  uint32_t val2;
} Dummy;

A series of calculations are performed thousands of times from both the main and audio thread, involving all the members. Without locks the steps are interleaved, resulting in the wrong result at the end.

It simulates a queue, for example, or any other container. Comment out the emscripten_lock_busyspin_wait_acquire() call and it will fail (with a shorter number of iterations sometimes it would get lucky, but running intensively for a few seconds ensures failure without locks).