Make a test suite

[munkm]

Right now the widgyts project is untested. We need to come up with a way to test at minimum some basic functionality of the package. I propose a suite that:

• checks to see that the dashboard is generated if a call is made to generate it
• checks to see what's returned to display if yt is loaded and not loaded
• checks that trait linking is working properly if two fields are loaded (i.e. try to update a trait on one field and ensure it is the same in the other field's traits)
• typechecking of widgyt objects

I'm not sure if we want to do rust + js + python testing or not? That seems like it's going to be fairly difficult so I think starting with what's synced to python seems like a good start and we can build out more from there.

[matthewturk]

There are tests in the rust package, and it's independent of this, so I think we only need to test the widgets in whatever mechanism Jupyter widgets are generally tested.

Not sure we need to do any typechecking, but maybe I'm just not understanding.