tests: create tests for various traffic generators on NULL

tests: create tests for various traffic generators on NULL

New tests were added for:

- garnet_synth_traffic.py
- ruby_random_test.py
- ruby_direct_test.py

These tests are factored out with the tests for:

- memcheck.py
- ruby_mem_test.py

Each new test was calibrated to last about 5 seconds on a Lenovo ThinkPad
P51.

Change-Id: Ie13d3963e1163cffae5dce329b623fbebbf2b983
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26808
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>