Move all e2e-tests related code into main repo

[brbrr]

There few repositories where we store test-related code. For example https://github.com/Automattic/wp-e2e-tests-for-branches/blob/master/wait-for-running-branch.sh. There also bridge servers, which tightly coupled with our e2e repo.

That's fine in most cases when you not touching any of ENV variables or constants. But for other cases, one can forget to check other repos, which will lead to breakage. And, in general, it's hard to feel confident changing some of the infrastructure configurations while you can't easily access and review all the pieces