Running the test for 12h is too long in some cases. It would be better if
the duration of the test could be set from outside. With the new environment
variable DURATION a user can control the value of the parameter `-D` of
cyclictest and set the desired value.
- Generate a latency plot PNG file in /tmp/latency-plot.png.
- Generate a latency summary file in /tmp/latency-summary.log.
Related-to: TOR-1198
Signed-off-by: Ming Liu <ming.liu@toradex.com>
