t/latency_percentiles: test latency percentile reporting

t/latency_percentiles: test latency percentile reporting

This script runs fio jobs exercising the slat_percentiles,
clat_percentiles, and lat_percentiles options under various conditions.
It carries out several tests comparing write_lat_log data with
percentiles reported by fio.

It also includes limited tests of high/low priority latency reporting
with the cmdprio_percentage option. Only Linux supports the
cmdprio_percentage option. Skips these tests on other platforms.

Signed-off-by: Vincent Fu <vincent.fu@wdc.com>