[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/table_spec.jsx

@@ -1,4 +1,3 @@
-import { expect } from 'chai';
 import $ from 'jquery';
 import '../../helpers/shim';
 import tableVis from '../../../src/visualizations/Table/adaptor';
@@ -33,24 +32,24 @@ describe('table viz', () => {
   it('renders into a container', () => {
     $('body').html(div);
     const container = $(baseSlice.selector);
-    expect(container.length).to.equal(1);
+    expect(container).toHaveLength(1);
   });
 
   it('renders header and body datatables in container', () => {
     $('body').html(div);
     const container = $(baseSlice.selector);
 
-    expect(container.find('.dataTable').length).to.equal(0);
+    expect(container.find('.dataTable')).toHaveLength(0);
     tableVis(baseSlice, basePayload);
-    expect(container.find('.dataTable').length).to.equal(2);
+    expect(container.find('.dataTable')).toHaveLength(2);
 
     const tableHeader = container.find('.dataTable')[0];
-    expect($(tableHeader).find('thead tr').length).to.equal(1);
-    expect($(tableHeader).find('th').length).to.equal(2);
+    expect($(tableHeader).find('thead tr')).toHaveLength(1);
+    expect($(tableHeader).find('th')).toHaveLength(2);
 
     const tableBody = container.find('.dataTable')[1];
-    expect($(tableBody).find('tbody tr').length).to.equal(2);
-    expect($(tableBody).find('th').length).to.equal(2);
+    expect($(tableBody).find('tbody tr')).toHaveLength(2);
+    expect($(tableBody).find('th')).toHaveLength(2);
   });
 
   it('hides the sort by column', () => {
@@ -74,7 +73,7 @@ describe('table viz', () => {
 
     const container = $(slice.selector);
     const tableHeader = container.find('.dataTable')[0];
-    expect($(tableHeader).find('th').length).to.equal(2);
+    expect($(tableHeader).find('th')).toHaveLength(2);
   });
 
   it('works with empty list for sort by', () => {
@@ -96,6 +95,6 @@ describe('table viz', () => {
 
     const container = $(slice.selector);
     const tableBody = container.find('.dataTable')[1];
-    expect($(tableBody).find('th').length).to.equal(3);
+    expect($(tableBody).find('th')).toHaveLength(3);
   });
 });