[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/visualizations/models/ChartPlugin_spec.js

@@ -1,5 +1,3 @@
-import { describe, it } from 'mocha';
-import { expect } from 'chai';
 import ChartPlugin from '../../../../src/visualizations/core/models/ChartPlugin';
 import ChartMetadata from '../../../../src/visualizations/core/models/ChartMetadata';
 
@@ -7,7 +5,7 @@ describe('ChartPlugin', () => {
   const metadata = new ChartMetadata({});
 
   it('exists', () => {
-    expect(ChartPlugin).to.not.equal(undefined);
+    expect(ChartPlugin).toBeDefined();
   });
 
   describe('new ChartPlugin()', () => {
@@ -16,13 +14,13 @@ describe('ChartPlugin', () => {
         metadata,
         Chart() {},
       });
-      expect(plugin).to.be.instanceof(ChartPlugin);
+      expect(plugin).toBeInstanceOf(ChartPlugin);
     });
     it('throws an error if metadata is not specified', () => {
-      expect(() => new ChartPlugin()).to.throw(Error);
+      expect(() => new ChartPlugin()).toThrowError(Error);
     });
     it('throws an error if none of Chart or loadChart is specified', () => {
-      expect(() => new ChartPlugin({ metadata })).to.throw(Error);
+      expect(() => new ChartPlugin({ metadata })).toThrowError(Error);
     });
   });
 
@@ -32,11 +30,11 @@ describe('ChartPlugin', () => {
       Chart() {},
     });
     it('throws an error if key is not provided', () => {
-      expect(() => plugin.register()).to.throw(Error);
-      expect(() => plugin.configure({ key: 'abc' }).register()).to.not.throw(Error);
+      expect(() => plugin.register()).toThrowError(Error);
+      expect(() => plugin.configure({ key: 'abc' }).register()).not.toThrowError(Error);
     });
     it('returns itself', () => {
-      expect(plugin.configure({ key: 'abc' }).register()).to.equal(plugin);
+      expect(plugin.configure({ key: 'abc' }).register()).toBe(plugin);
     });
   });
 });