WIP: Execute review app tests in parallel via the parallel_tests gem

What does this MR do?

Extends https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29654 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30113 by enabling the --parallel option for review apps.

This shouldn't be merged until after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30113 when we're confident the tests can safely run in parallel.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content