feat: use create-jest-runner

[SimenB]

This simplifies the code a great deal.

create-jest-runner also contains helpers for the boilerplate around creating test results, but I didn't do anything with them for now.

---

I have high hopes that if we get this module to be smooth people can use it to typecheck and transile their code, and use Babel for Jest (and maybe https://github.com/rickhanlonii/jest-runner-babel for transpiling, leaving this module to just do type-checking in the future)

/cc @rogeliog @rickhanlonii

[SimenB]

We'll need to land runner config at some point (https://github.com/facebook/jest/issues/4278), so people can provide you with the path to their tsconfig file etc.

[azz]

Thanks! That's so much cleaner!