Re-evaluate adding back test pipeline parallel jobs

In lieu of https://gitlab.com/gitlab-com/gl-infra/production/-/issues/20083 brought upon due to failed test resource cleanup and suspected increased jobs causing database and general environment instability in staging, this issue is to evaluate post-revert of gitlab-org/quality/pipeline-common!526 (merged) whether we're better poised to re-add parallel jobs back to have each running under 15 minutes again.