[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/makeSingleton_spec.js

@@ -1,5 +1,3 @@
-import { describe, it } from 'mocha';
-import { expect } from 'chai';
 import makeSingleton from '../../../src/utils/makeSingleton';
 
 describe('makeSingleton()', () => {
@@ -14,24 +12,30 @@ describe('makeSingleton()', () => {
   describe('makeSingleton(BaseClass)', () => {
     const getInstance = makeSingleton(Dog);
 
-    it('returns a function for getting singleton instance of a given base class', () => {
-      expect(getInstance).to.be.a('Function');
-      expect(getInstance()).to.be.instanceOf(Dog);
-    });
+    it(
+      'returns a function for getting singleton instance of a given base class',
+      () => {
+        expect(typeof getInstance).toBe('function');
+        expect(getInstance()).toBeInstanceOf(Dog);
+      },
+    );
     it('returned function returns same instance across all calls', () => {
-      expect(getInstance()).to.equal(getInstance());
+      expect(getInstance()).toBe(getInstance());
     });
   });
   describe('makeSingleton(BaseClass, ...args)', () => {
     const getInstance = makeSingleton(Dog, 'Doug');
 
-    it('returns a function for getting singleton instance of a given base class constructed with the given arguments', () => {
-      expect(getInstance).to.be.a('Function');
-      expect(getInstance()).to.be.instanceOf(Dog);
-      expect(getInstance().name).to.equal('Doug');
-    });
+    it(
+      'returns a function for getting singleton instance of a given base class constructed with the given arguments',
+      () => {
+        expect(typeof getInstance).toBe('function');
+        expect(getInstance()).toBeInstanceOf(Dog);
+        expect(getInstance().name).toBe('Doug');
+      },
+    );
     it('returned function returns same instance across all calls', () => {
-      expect(getInstance()).to.equal(getInstance());
+      expect(getInstance()).toBe(getInstance());
     });
   });