Enable parallelisation in test-on-omnibus

Scope

The scope of this issue is to enable the implementation of parallelization in the omnibus pipeline running in MRs. The goal is to ensure the implementation does not increase the retry rate and to achieve test runtime reduction (currently targeted at 25% per OKR). The completion criteria of this issue is to activate in 1 single scenario. A rollout follow-up is tracked in Parallelisation rollout strategy.

Before Parallel

Pipeline: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1436062284 -> 140 minutes 31 seconds

After Parallel

placeholder for final results when the implementation is deployed and enabled.