Snapshot testing

[zenangst]

I've grown rather fond of snapshot tests lately and we might want to consider including http://github.com/pointfreeco/swift-snapshot-testing on the unit test target so that we can rely on their dump-based asserts when writing our tests.

And it goes without saying that we should use it for the ViewKit target to get snapshots of views and windows. I think that could be achieved by simply loading the _previews and creating a snapshot from a NSHostingController.

[vadymmarkov]

Yes, this is a good idea!

[zenangst]

It supports SPM so I think we should go ahead and try and include it as a dependency.
I think #14 could benefit from using snapshot testing.

[zenangst]

This is fixed in #16