Implement Browser.on(event)

[imiric]

This is an implementation of Browser.on(event), using the recently released event loop support in k6. The only supported event is disconnected, so it can be used as an alternative to teardown().

It also does a few minor fixes and refactors. See the commits for details.

Closes #95

[inancgumus]

Here are my suggestions.

[inancgumus]

@imiric, by the way, we don't return here even though the connection is closed:

func (b *Browser) initEvents() error {
	...
	go func() {
		for {
			select {
			case <-cancelCtx.Done():
				return
			case event := <-chHandler:
				...
				} else if event.typ == EventConnectionClose {
					...
					b.browserProc.didLoseConnection()
					b.cancelFn()

					// LOOK HERE: it makes sense to return here
				}
			}
		}
	}()

This might be a bug (even though the next iteration will catch that the context is being closed). I wanted to let you know.

[inancgumus]

Nice job.