Automatic parallelization of tests

While he have CI runners that are able to operate in distributed environment and run test suites in parallel, an interesting idea may be introducing mechanism to split test suites even further and run test even faster.

It may be particularly useful for large organizations that have test suites running for hours. This would require introducing some kind of abstraction that may be implemented using various programming languages (like PHP, Java, C++). This implementation should be ten consumed somehow by GitLab CI/runners.

There are few existing projects that are using this approach, for e.g. https://github.com/grosser/parallel_tests, but we need something more versatile.

This may be an interesting idea, but this is also a tough one, and needs a lot of thoughts.

\cc @sytses, @ayufan, @dzaporozhets, @JobV