This change migrates all the existing unit tests
- to Jest's global expect and matchers from chai's imported expect, asserts and matchers.
- to Jest's describe/test from mocha's describe/it

The majority of the mechanical changes to tests are achieved through running jest-codemods. The only two note-worthy manual tweaks:
1. Setting a testURL of http://localhost in jest config and adjusting a few tests to leverage this value instead of relying on about:blank.
2. Re-enabling ExploreChartPanel_spec which was previously commented out as we cannot have empty tests with nothing in it with Jest. :)

This change also removes dependencies to Mocha and Chai.

…arn run lint --fix`

The only noteworthy change is the one in eslintrc for tests. The env has been updated from mocha to jest.

- One small fix in sqllab's Timer Spec for a test that is not using the spy it created for testing.
- Deletion of a duplicated test caught by eslint-plugin-jest.

- Remove dependency on stand-alone istanbul and babel-istanbul as they're built-into jest. Yes!

… codecov

- remove dynamic import in shim.js now that it is set in babelrc for tests only.