Minimal Integration Testing

As the current state of #18 unit testing is rather minimal, we should likely consider investing more effort in establishing a reasonable, locally-replicable integration testing approach.

## Options
- [galata](https://github.com/jupyterlab/galata)
- pro:
- official jupyterlab subproject
- js/ts-extensible
- builds on jest ecosystem
- con:
- about two weeks old
- requires ts/js knowledge
- no particularly viable way to write in lab (`jupyterlab-lsp` makes this.... better, but js/ts have lots of sharp edges)
- [robotframework-jupyterlibrary](https://github.com/robots-from-jupyter/robotframework-jupyterlibrary)
- pro:
- maintained over about two years
- builds on robotframework ecosystem
- python and robot extensible
- plain-english test cases and user keywords
- can be authored in-lab with `robotkernel` or `xeus-robot`
- con:
- single maintainer (me)
- robot syntax is a little funny the first couple times