tests,misc: Updated presubmit YAML to use main.py '-t' flag

This minor change reduces the presubmit build time by about 10 to 15
minutes.

Change-Id: I3a87d1a720b17fd22a9dbdbeebfb32e4be178c56
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27064
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

cloudbuild_presubmit.yaml

@@ -23,7 +23,7 @@ steps:
     - name: 'gcr.io/$PROJECT_ID/ubuntu-18.04_all-dependencies:latest'
       dir: 'tests'
       entrypoint: 'python'
-      args: ['main.py', 'run', '-j', '4', '--length', 'quick']
+      args: ['main.py', 'run', '-j', '4', '-t', '8', '--length', 'quick']
 
 images: ['gcr.io/$PROJECT_ID/ubuntu-18.04_all-dependencies:latest']
 options: