Write unit tests for IPyWidgets

[DonJayamanne]

Instead of running notebooks and rendering widgets, we might want to write some test that

• Tests state synchronization
• Tests message transfers back n forth
• Test model syncing

Basically we need a protocol level test akin to lsp/debugger protocol tests, instead of full end to end tests.
This would require some much needed refactor with the following benefits

• Better and simpler tests
• Npm package or the like for ease of use of widgets (in other webviews)

[DonJayamanne]

Closing as this will happen as part of re-factoring widgets.