[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/components/DashboardGrid_spec.jsx

@@ -1,6 +1,5 @@
 import React from 'react';
 import { shallow } from 'enzyme';
-import { expect } from 'chai';
 import sinon from 'sinon';
 
 import DashboardComponent from '../../../../src/dashboard/containers/DashboardComponent';
@@ -31,39 +30,37 @@ describe('DashboardGrid', () => {
 
   it('should render a div with class "dashboard-grid"', () => {
     const wrapper = setup();
-    expect(wrapper.find('.dashboard-grid')).to.have.length(1);
+    expect(wrapper.find('.dashboard-grid')).toHaveLength(1);
   });
 
   it('should render one DashboardComponent for each gridComponent child', () => {
     const wrapper = setup({
       gridComponent: { ...props.gridComponent, children: ['a', 'b'] },
     });
-    expect(wrapper.find(DashboardComponent)).to.have.length(2);
+    expect(wrapper.find(DashboardComponent)).toHaveLength(2);
   });
 
   it('should render two empty DragDroppables in editMode to increase the drop target zone', () => {
     const viewMode = setup({ editMode: false });
     const editMode = setup({ editMode: true });
-    expect(viewMode.find(DragDroppable)).to.have.length(0);
-    expect(editMode.find(DragDroppable)).to.have.length(2);
+    expect(viewMode.find(DragDroppable)).toHaveLength(0);
+    expect(editMode.find(DragDroppable)).toHaveLength(2);
   });
 
   it('should render grid column guides when resizing', () => {
     const wrapper = setup({ editMode: true });
-    expect(wrapper.find('.grid-column-guide')).to.have.length(0);
+    expect(wrapper.find('.grid-column-guide')).toHaveLength(0);
 
     wrapper.setState({ isResizing: true });
 
-    expect(wrapper.find('.grid-column-guide')).to.have.length(
-      GRID_COLUMN_COUNT,
-    );
+    expect(wrapper.find('.grid-column-guide')).toHaveLength(GRID_COLUMN_COUNT);
   });
 
   it('should render a grid row guide when resizing', () => {
     const wrapper = setup();
-    expect(wrapper.find('.grid-row-guide')).to.have.length(0);
+    expect(wrapper.find('.grid-row-guide')).toHaveLength(0);
     wrapper.setState({ isResizing: true, rowGuideTop: 10 });
-    expect(wrapper.find('.grid-row-guide')).to.have.length(1);
+    expect(wrapper.find('.grid-row-guide')).toHaveLength(1);
   });
 
   it('should call resizeComponent when a child DashboardComponent calls resizeStop', () => {
@@ -73,8 +70,8 @@ describe('DashboardGrid', () => {
     const dashboardComponent = wrapper.find(DashboardComponent).first();
     dashboardComponent.prop('onResizeStop')(args);
 
-    expect(resizeComponent.callCount).to.equal(1);
-    expect(resizeComponent.getCall(0).args[0]).to.deep.equal({
+    expect(resizeComponent.callCount).toBe(1);
+    expect(resizeComponent.getCall(0).args[0]).toEqual({
       id: 'id',
       width: 1,
       height: 3,