Stop using Jest, use Mocha instead

[qntm]

I discovered that Jest is ridiculously slow to load and run tests by comparison with Mocha. This isn't significant for t-a-i where Jest takes ~4.8 seconds to run all the tests and Mocha takes more like ~1.6 end-to-end (that's including the startup time for npm in both cases), but for some other projects of mine (including at work) this could be significant, saving multiple minutes of execution, a dozen times per day. So, this is in part an exercise to explore the difficulties of a Jest-to-Mocha migration.

• Stop using Jest's expect assertion functionality in favour of Node.js's built-in assert module.
  • This is where a lot of questions come in. This could potentially be difficult to do in the general case. Luckily, t-a-i doesn't use much of expect's more advanced functionality.
• Install Mocha and (for coverage) c8.
• Use c8 ignores for code coverage.
• Move all test files over to a dedicated test directory, where Mocha expects them to be by default.
• I guess we no longer need to manually exclude those test files from our "files" anymore, that's nice.
• Scrap the Jest config file.
• Rewrite our npm scripts to use Mocha, not Jest.
• Eliminate Jest as a development dependency.
• (Unrelated) Get rid of Stryker and give up on the mutation testing thing. It was fun to visit but it doesn't seem practical to reach 100% coverage which makes it not so useful for me.