Enable parallel_tests for test-on-cng pipeline

Closes gitlab-org/quality/quality-engineering/team-tasks#3034

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167719



Merged-by: Andrejs Cunskis <acunskis@gitlab.com>
Approved-by: David Dieulivol <ddieulivol@gitlab.com>
Approved-by: Sofia Vistas <svistas@gitlab.com>
Reviewed-by: Andrejs Cunskis <acunskis@gitlab.com>