Add a test suite

[scottyantipa]

Experimenting with different test runners in these PRs:

• jest Setup Jest for testing #49
• mocha Add mocha as a test runner #50

Honorary mention to https://github.com/avajs/ava for simplicity and Magic Diffs

[scottyantipa]

Unfortunately I haven't found a way around Jest being quite slow, especially for a small number of tests.

Some existing issues about slowness which are still open, and stale:

• Jest performance is at best 2x slower than Jasmine, in our case 7x slower jestjs/jest#6694
• Very poor performance on basic tests jestjs/jest#7963
• https://itnext.io/how-to-make-your-sluggish-jest-v23-tests-go-faster-1d4f3388bcdd

These are the stats I got for the small test suite implemented in #49 and #50

|Speed comparison
|Mocha:
|- test:cover in 4.3s (also checking too many files)
|- test in 2.1s
|- --watch feels instantaneous
|Jest:
|- test:cover in 4.8s
|- test in 4.6s
|- --watch is far from instantaneous. Takes at least 2s.

[scottyantipa]

Some visual comparison on test outputs. I'd say they're about the same, although jest has a nicer way of displaying the failed source line clearly.

Obj diffs:
Jest:

Mocha

and AVA for fun:

Test failure output
Jest:

Mocha

[scottyantipa]

One very nice feature of jest that mocha lacks is that it will only re-run tests that have changed in the current commit while in --watch mode. Whereas mocha will seemingly re-run the entire test suite, which could get slow.