Add direct tests for the generic event loop

[camshaft]

Problem:

In #1740, we added a generic event loop that replaces the existing ones that have been copy/pasted for each IO provider. This new code is indirectly tested through the io/testing IO provider and all of the associated integration tests.

Solution:

It would be nice to increase coverage by adding some direct tests to the code in order to state all of the properties that it needs to hold to be correct. This should include at least some unit tests to show the event loop waking up and emitting the correct events to the publisher. Ideally, it would also include a single iteration bolero/kani test.