dts: resolve variance in performance reporting

Currently, the single core forward performance test suite reports
variable results with a standard deviation of ~10Mpps. Currently,
traffic is measured over 30 seconds a single time and the stats are
reported from this metric. This commit reduces the time traffic is sent
to only 5 seconds but runs numerous times to take an average and limit
variance to a standard deviation of ~1Mpps.

Bugzilla ID: 1969
Fixes: d77d7f04f24c ("dts: add single-core performance test suite")
Cc: stable@dpdk.org

Signed-off-by: Andrew Bailey <abailey@iol.unh.edu>