tests/p4wq: Fix impossible sleep interval

The code here was written to "get out of the way just long enough for
the trivial context switch and callback to execute".  But on a machine
with 50 kHz ticks, that's not reliably enough time and this was
failing spuriously.  Which would have been a reasonably forgivable
mistake to make had I not written this code with this very machine in
mind...

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>