`expect(chrome.runtime.onMessage.addListener).toHaveBeenCalled()`

### Is your feature request related to a problem? Please describe.

I'm trying to test that a function is registering events, but it seem that this use case was not considered. The readme helpfully demoes the testing of events but not in the same way.

### Describe the solution you'd like

`addListener` should be mocked so that I can `expect` calls on it