Tests!

[parente]

We have tests for the backend components. We have a way to test the frontend too (see dashboards / declarative widgets). Let's get something going as a safety net at least.

• bundler.py
• loader.py and inject.py
• index.py and search.py
• frontend tests? (worth it if features will one day be in JupyterLab?)

[parente]

Did add tests for bundler tool as part of #15 to start.

[parente]

Added tests for loader.py and inject.py in #22.

Removed the milestone because more will come over time. Don't need to do them all at once.