[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/utils/convertKeysToCamelCase_spec.js

@@ -1,13 +1,11 @@
-import { it, describe } from 'mocha';
-import { expect } from 'chai';
 import convertKeysToCamelCase from '../../../src/utils/convertKeysToCamelCase';
 
 describe('convertKeysToCamelCase(object)', () => {
   it('returns undefined for undefined input', () => {
-    expect(convertKeysToCamelCase(undefined)).to.equal(undefined);
+    expect(convertKeysToCamelCase(undefined)).toBeUndefined();
   });
   it('returns null for null input', () => {
-    expect(convertKeysToCamelCase(null)).to.equal(null);
+    expect(convertKeysToCamelCase(null)).toBeNull();
   });
   it('returns a new object that has all keys in camelCase', () => {
     const input = {
@@ -15,15 +13,15 @@ describe('convertKeysToCamelCase(object)', () => {
       'is-angry': false,
       isHungry: false,
     };
-    expect(convertKeysToCamelCase(input)).to.deep.equal({
+    expect(convertKeysToCamelCase(input)).toEqual({
       isHappy: true,
       isAngry: false,
       isHungry: false,
     });
   });
   it('throws error if input is not a plain object', () => {
-    expect(() => { convertKeysToCamelCase({}); }).to.not.throw();
-    expect(() => { convertKeysToCamelCase(''); }).to.throw();
-    expect(() => { convertKeysToCamelCase(new Map()); }).to.throw();
+    expect(() => { convertKeysToCamelCase({}); }).not.toThrowError();
+    expect(() => { convertKeysToCamelCase(''); }).toThrowError();
+    expect(() => { convertKeysToCamelCase(new Map()); }).toThrowError();
   });
 });