Reintroduce parallel testing for E2E pipelines

What does this MR do and why?

Activates and improves the parallel testing implementation. This a second iteration (after it first got introduced), focusing on test-on-gdk, test-on-cng and package-and-test pipelines. Note that not all scenarios have been parallelized in this iteration and there is a current trade-off on switching off failure videos. These seem to be incompatible, yet require more investigation.

This MR introduces a 35% reduction in total pipeline duration.

Related epic: gitlab-org/quality/quality-engineering&69