[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/gridComponents/Chart_spec.jsx

@@ -1,6 +1,5 @@
 import React from 'react';
 import { shallow } from 'enzyme';
-import { expect } from 'chai';
 import sinon from 'sinon';
 
 import Chart from '../../../../../src/dashboard/components/gridComponents/Chart';
@@ -50,39 +49,39 @@ describe('Chart', () => {
 
   it('should render a SliceHeader', () => {
     const wrapper = setup();
-    expect(wrapper.find(SliceHeader)).to.have.length(1);
+    expect(wrapper.find(SliceHeader)).toHaveLength(1);
   });
 
   it('should render a ChartContainer', () => {
     const wrapper = setup();
-    expect(wrapper.find(ChartContainer)).to.have.length(1);
+    expect(wrapper.find(ChartContainer)).toHaveLength(1);
   });
 
   it('should render a description if it has one and isExpanded=true', () => {
     const wrapper = setup();
-    expect(wrapper.find('.slice_description')).to.have.length(0);
+    expect(wrapper.find('.slice_description')).toHaveLength(0);
 
     wrapper.setProps({ ...props, isExpanded: true });
-    expect(wrapper.find('.slice_description')).to.have.length(1);
+    expect(wrapper.find('.slice_description')).toHaveLength(1);
   });
 
   it('should call refreshChart when SliceHeader calls forceRefresh', () => {
     const refreshChart = sinon.spy();
     const wrapper = setup({ refreshChart });
     wrapper.instance().forceRefresh();
-    expect(refreshChart.callCount).to.equal(1);
+    expect(refreshChart.callCount).toBe(1);
   });
 
   it('should call addFilter when ChartContainer calls addFilter', () => {
     const addFilter = sinon.spy();
     const wrapper = setup({ addFilter });
     wrapper.instance().addFilter();
-    expect(addFilter.callCount).to.equal(1);
+    expect(addFilter.callCount).toBe(1);
   });
 
   it('should return props.filters when its getFilters method is called', () => {
     const filters = { column: ['value'] };
     const wrapper = setup({ filters });
-    expect(wrapper.instance().getFilters()).to.equal(filters);
+    expect(wrapper.instance().getFilters()).toBe(filters);
   });
 });