test-process-util: run fewer getpid() tests

test-process-util: run fewer getpid() tests

Significant time was spent in the getpid() measurement code, which is not very
important.  So let's optimize this a bit by running the slower version less
times, and only running both tests a lesser amount of times unless slow tests
are enabled.

This gives the better accuracy then before in slow mode, and still reasonable
accuracy in fast mode without a noticable slowdown.