test - Flesh out randread statistics reporting

test - Flesh out randread statistics reporting

* Cleanup the output.  Report the aggregate IOPS, average latency,
  aggregate bandwidth, low and high latencies, and the standard
  deviation for the test on a per-second basis.

* Standard deviation notes:

  68%	    Of the samples are within 1 standard deviation, so if the
	    reported stddev is 7uS, then 68% of the samples are +/- 7uS
	    of the average reported value.

  95.45%    Of the samples are within 2 standard deviations.  +/- 14uS
	    in the example.

  99.73%    Of the samples are within 3 standard deviations.  +/- 21uS
	    in the example.