nsleep-lat: Add better timer latency checks

Improve the nanosleep latency test by adding absolute timer latency
checks and increasing the timer length each iteration.

Also rename the test to match that its a general latency test now.

Signed-off-by: John Stultz <john.stultz@linaro.org>
set-2038: Add nanosleep latency check to validation

Call the nsleep-lat test as well when validating date corner cases.

Signed-off-by: John Stultz <john.stultz@linaro.org>