Test multiple event handlers

[hjdivad]

@Cyril-sf I have megadoubts about https://github.com/Cyril-sf/MessageChannel.js/blob/master/lib/message_channel.js#L354-L388

We talked about this earlier: I think the following case will fail

1. add event handler A
2. add event handler B
3. remove event handler A
4. fire message event; handler A should not fire.

Let's at least test the multihandler case, although I think we'll need a little bit of work to support it as well.

[hjdivad]

@Cyril-sf n/m I see how that's supposed to work now. Still, removing event handlers in multihandler cases is not tested and it should be.

We may want to make the wrapper less intrusive: at least if defineProperty exists, we can make it non-enumerable.

[cyril-sf]

As discussed, I'll add an explicit test. This is "tested" when tearing down the tests.

Also, the current implementation won't support the fact that we could theoretically add several times the same handler.