Don't use a fixed iteration count in dist/threads-shared/t/stress.t

Looping 500,000 times takes between 0.025s and 1s depending on hardware and
optimisation levels on machines I have access to. For a fixed iteration
count, on a particularly slow machine the timeout can fire before all
threads have had a realistic chance to complete, but dropping the iteration
count will cause fast machines to finish each thread too quickly.

So use an initial busy loop (single-thread) to estimate a suitable iteration
count to use for the per-thread test loop.

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