Use Jest for testing

[rkalis]

After the call last week I decided to see if I could get Jest up and running.

[rkalis]

I was able to set up Jest for testing, replacing mocha+chai+nyc. The process was pretty straightforward:

1. Uninstall Mocha + Chai + nyc.
2. Install Jest + ts-jest.
3. Consolidate mocha.opts + .nycrc into jest.config.js and jest.setup.js.
4. Add env.jest = true to .erclintrc.js.
5. Update test files to use Jest matchers instead of Chai matchers/assertions.

Then there was one gotcha when using Jest with Bitbox:

1. Bitbox relies on Buffer instanceof Uint8Array which is expected behaviour. But this breaks with Jest (Jest breaks Buffer instanceof Uint8Array jestjs/jest#4422). The IPFS guys created a custom environment that fixes this (jest-environment-aegir). This code is outdated though, so I updated the code and added it to my own project (jest/custom-environment.js). If you're using Jest with any project that depends on Bitbox, this is a must.

And some optional extra steps I took:

1. I added extra reporters for cleaner test output.
2. To make better use of Jest's concurrency I split up some test files into multiple files.