(refactor/test): switch test env to jest w/ electron runner

- use a full browser for testing instead of running in Node and
  relying on JSDOM, which does not implement all of the features we
  use (namely worker-related stuff)
  - this ensures tests run without any errors
  - and @jest-runner/electron is surprisingly **very fast**, not much
    different from running in normal jest w/ jsdom
    - like 4s locally fast I mean

- refactor tests to Jest style from AVA style
- remove all AVA-specifc config

Author: agilgur5

jest.config.js

@@ -0,0 +1,7 @@
+module.exports = {
+  runner: '@jest-runner/electron',
+  testEnvironment: '@jest-runner/electron/environment',
+  // don't need to run babel on anything
+  // (and browserify bundle takes very long and sometimes errors out if we do)
+  transform: {}
+}