tests/kernel/sleep: relax upper bound for TI CC13X2/CC26X2 RTC

The RTC on TI CC13X2/CC26X2 is a 32 KHz clock for which the minimum
compare delay is 3 ticks. When using it as the system clock, we need
to relax the upper bound to ensure the test succeeds.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>