Livecap tests

Here's something I was wanting - capturing on multiple interfaces - and a test for LiveCapture.

This can be a first draft - I made it such that if you specify `LiveCapture(interface="all")` you capture on all interfaces.

However, another solution might be to make another keyword argument for `LiveCapture.__init__()` like `interface_list` that expects a list of interfaces. There would probably still need to be special logic to handle capturing from all interfaces.

The test could possibly use some work - getting some packets from something in the standard library was the goal. It passes on my machine, at least.