[refactor] Migrate from Mocha+Chai to Jest (#6079)

* [refactor] Migrate from Mocha+Chai to Jest

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.

* Remove the test:one command as it now does the same thing as test.

* Fixing lint errors. The diff looks large but is large done through `yarn run lint --fix`

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

* Adding eslint-plugin-jest and further modify tests.

- 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.

* - Make istanbul coverage work with Jest.

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

* Attempt to fix dynamic imports in tests.

* run sequentially and log heap usage

* - tweaking maxworkers for travis and specifying coverageDirectory for codecov

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

superset/assets/spec/javascripts/explore/components/DisplayQueryButton_spec.jsx

@@ -1,5 +1,4 @@
 import React from 'react';
-import { expect } from 'chai';
 import { mount } from 'enzyme';
 import ModalTrigger from './../../../../src/components/ModalTrigger';
 
@@ -20,10 +19,10 @@ describe('DisplayQueryButton', () => {
   };
 
   it('is valid', () => {
-    expect(React.isValidElement(<DisplayQueryButton {...defaultProps} />)).to.equal(true);
+    expect(React.isValidElement(<DisplayQueryButton {...defaultProps} />)).toBe(true);
   });
   it('renders a dropdown', () => {
     const wrapper = mount(<DisplayQueryButton {...defaultProps} />);
-    expect(wrapper.find(ModalTrigger)).to.have.lengthOf(3);
+    expect(wrapper.find(ModalTrigger)).toHaveLength(3);
   });
 });