[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/dashboard/util/componentIsResizable_spec.js

@@ -1,5 +1,3 @@
-import { expect } from 'chai';
-
 import componentIsResizable from '../../../../src/dashboard/util/componentIsResizable';
 import {
   CHART_TYPE,
@@ -29,13 +27,13 @@ const resizable = [COLUMN_TYPE, CHART_TYPE, MARKDOWN_TYPE];
 describe('componentIsResizable', () => {
   resizable.forEach(type => {
     it(`should return true for ${type}`, () => {
-      expect(componentIsResizable({ type })).to.equal(true);
+      expect(componentIsResizable({ type })).toBe(true);
     });
   });
 
   notResizable.forEach(type => {
     it(`should return false for ${type}`, () => {
-      expect(componentIsResizable({ type })).to.equal(false);
+      expect(componentIsResizable({ type })).toBe(false);
     });
   });
 });