Component Unit Tests

[mjlangan]

The huge line change counts are from updating yarn.lock 😄

This PR does a few things with the ultimate goal of adding component unit tests to the codebase:

• Switched from jest to mocha, because most other projects here at AI2 use mocha
• Added enzyme for component unit testing and sinon for mocks
• Downgraded React from 17 to 16 for two reasons: firstly, enzyme doesn't yet support React 17. secondly, S2 uses React 16, so now this is the same major version.
• Added JSDOM for running component unit tests without a browser
• Disabled allowSyntheticDefaultImports in tsconfig because mocha does not like those imports (and you get code that works in the browser/webpack but not in tests)
• Added tests for Overlay and BoundingBox
• Fixed Overlay requiring children
• Fixed BoundingBox's className assignment

[mjlangan]

Should the switch to mocha have any impact on how we write tests going forward?

There shouldn't be any real impacts. Assertions change slightly because jest provided its own expect implementation, and now it's using chai's, but this also brings things more in line with S2.