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>

tests/gem5/traffic_gen/simple_traffic_run.py

@@ -209,7 +209,9 @@ print("Beginning simulation!")
 exit_event = m5.simulate()
 print(f"Exiting @ tick {m5.curTick()} because {exit_event.getCause()}.")
 
-simstats = get_simstat(root, prepare_stats=True)
+simstats = get_simstat(
+    [core.generator for core in generator.get_cores()], prepare_stats=True
+)
 json_output = Path(m5.options.outdir) / "output.json"
 with open(json_output, "w") as stats_file:
     simstats.dump(stats_file, indent=2)