GitLab

This adds the tests on the GN bots that are currently configured
recipe-side in the chromium_gn recipe to //testing/scripts, so
that they can be configured repo-side and we will need to modify
the recipe less often.

R=phajdan.jr@chromium.org
BUG=481693

Review URL: https://codereview.chromium.org/1145513005

Cr-Commit-Position: refs/heads/master@{#330862}