stop waithires.t failing under high load

stop waithires.t failing under high load

In threads::shared, the waithires.t test checks cond_timedwait() with
sub-second timeouts. If the newly-minted child doesn't manage to grab the
lock within 0.05s, the cond_timedwait() will timeout, and the child and
the test will hang, until eventually killed off by the watchdog.  This can
easily happen on a slow/loaded system.

We fix this by putting the cond_timedwait() in a retry loop, only giving
up after 10 seconds of repeated timeouts.