Make event loop code testable in its module

[mstoykov]

Post #2228 we now have an event loop and that is great but nothing is using it. One of the problems that is making this harder is the fact currently the only actual way to test what happens is to run an integration test. This though is quite cumbersome and in practice requires the test to be outside the module implementing the code as otherwise there is every chance for an import cycle. This is even worse for extensions IMO.

Proposed solution:

Well the predominant thing missing is access to the event loop, so just moving it to a separate module and exporting fixes most of this IMO.

A current WIP PR can be seen at #2399

Other ideas:

We can only do the changes to modulestest and let implementors mock the event loop which might be a better first step. But I kind of feel having to reimplement the event loop in parts isn't a great idea ;)