Consider using `parallel_tests` to speed up the test suite on the CI

As suggested by @engwan in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12688#note_171255576:

Another idea I had was to use something like https://github.com/grosser/parallel_tests to parallelize our tests within a job so we could use a smaller number of jobs. There are docs on how to use this with knapsack: https://github.com/KnapsackPro/knapsack_pro-ruby#how-to-run-knapsack_pro-with-parallel_tests-gem

Rails 5.1.5 introduced parallel tests but we can't use that with RSpec so we'd need to use that gem.

This makes us fully utilize our runners and potentially save a lot of machine time.

If we have dual-core runners, we could theoretically cut our machine time by almost 50% by splitting to only 25 jobs and keep the running time almost the same.

That is huge and I think is worth an experiment to see how hard it is to get something like that setup.