test75: force child to use a minimum of cpu time

test75: force child to use a minimum of cpu time

test75 sometimes false-fails if something else is going on
at the same time, presumably because the child doesn't spin
enough to register a nonzero getrusage() cpu time value, as
spin() uses the real time to limit the spinning.

this change forces spin() to do a minimum amount of spinning
before exiting, regardless of scheduling.

Change-Id: I57c63d22969bba418f36bcc8c5ace2b6fb445968