Add vision direction for feature testing

What does this MR do and why?

I've created this MR to discuss our long-term vision on feature (end-to-end) tests in GitLab.

Please share your thoughts on the end-to-end testing framework we're currently using, and what we can imagine using in 3-5 years from now.

I would like to hightlight that we only consider a high-level vision. The exact framework, process (if any), etc. are out of scope of this MR.