test: add latency test using posixaio ioengine

test: add latency test using posixaio ioengine

Make sure that mean(slat) + mean(clat) = mean(total lat).

Tests 15 and 16 use the libaio and null ioengines, respectively. Both of
those ioengines have commit hooks. Add this new test using the posixaio
ioengine which does not have a commit hook so that we can better cover
the possible ways that latency is calcualted.

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