tests: kernel/sleep: increase maximum threshold for nRF51

nRF51 MCUs are Cortex-M0 running with a 16 MHz clock.  The overhead of
work done in k_usleep() requires adding three more ticks (92 us) to the
expected loop iteration time.  (Two ticks is enough on most boards, but
some require a little more time.)

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>