Reduce iterations and yield in multithreaded race tests

Under ThreadSanitizer (TSAN) on constrained CI runners, 1000 iterations
of font file parsing while reader threads spin without yielding caused
excessive lock contention, thread starvation, and test timeouts
exceeding 600 seconds.

Reduce NUM_ITERATIONS to 100 and add sched_yield() calls in spin-wait
and reader loops. This reduces TSAN execution time by orders of magnitude
while still reliably exercising concurrent reader and writer operations.

Changelog: none