tests: Reducing json stat dump size.

This change reduces the number of stats dumped as json in
traffic_gen tests.

Change-Id: I94becb2e6d5da6096271cf7893ff2b380314da06
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/71402
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>