Make busy wait busier

Another stab in the dark to fix the intermittent failures of timeout
tests on macos CI: We're using ITIMER_PROF, which means that the
timer counts against user+system time. The "busy" wait loop counts
against real time. Currently it calls microtime() on every iteration.
If that call is implemented as a syscall rather than going through
vDSO or commpage we might be seeing many context switches here which
drive up the real time, but not user or system time.

See if making the loop busier and calling microtime() less helps the
situation.

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