tests/kernel/sleep: relax upper bound to support additional delays

Nordic platforms measure ticks in cycles of a 32 KiHz clock for which
the minimum compare delay is 2 ticks.  The test assumed an upper bound
of four ticks delay per loop iteration, resulting from alignment at
various layers.  This delay is met on Nordic for tick rates at or below
16384, but is too short for the default 32768 Hz tick rate.

Instrumentation confirms that the usleep test loop body on Nordic at 32
KiHz ticks takes 3 ticks as the optimum delay, only when a debug probe
is active.  In other circumstances it can take either 5 or 6 ticks,
depending on timer alignment and stability.

Relax the upper bound for platforms using this system timer at the
highest rate.

Closes #17965.

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