[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/components/MetricOption_spec.jsx

@@ -1,5 +1,4 @@
 import React from 'react';
-import { expect } from 'chai';
 import { shallow } from 'enzyme';
 
 import MetricOption from '../../../src/components/MetricOption';
@@ -26,43 +25,43 @@ describe('MetricOption', () => {
     props = Object.assign({}, defaultProps);
   });
   it('is a valid element', () => {
-    expect(React.isValidElement(<MetricOption {...defaultProps} />)).to.equal(true);
+    expect(React.isValidElement(<MetricOption {...defaultProps} />)).toBe(true);
   });
   it('shows a label with verbose_name', () => {
     const lbl = wrapper.find('.option-label');
-    expect(lbl).to.have.length(1);
-    expect(lbl.first().text()).to.equal('Foo');
+    expect(lbl).toHaveLength(1);
+    expect(lbl.first().text()).toBe('Foo');
   });
   it('shows 3 InfoTooltipWithTrigger', () => {
-    expect(wrapper.find(InfoTooltipWithTrigger)).to.have.length(3);
+    expect(wrapper.find(InfoTooltipWithTrigger)).toHaveLength(3);
   });
   it('shows only 2 InfoTooltipWithTrigger when no descr', () => {
     props.metric.description = null;
     wrapper = shallow(factory(props));
-    expect(wrapper.find(InfoTooltipWithTrigger)).to.have.length(2);
+    expect(wrapper.find(InfoTooltipWithTrigger)).toHaveLength(2);
   });
   it('shows a label with metric_name when no verbose_name', () => {
     props.metric.verbose_name = null;
     wrapper = shallow(factory(props));
-    expect(wrapper.find('.option-label').first().text()).to.equal('foo');
+    expect(wrapper.find('.option-label').first().text()).toBe('foo');
   });
   it('shows only 1 InfoTooltipWithTrigger when no descr and no warning', () => {
     props.metric.warning_text = null;
     wrapper = shallow(factory(props));
-    expect(wrapper.find(InfoTooltipWithTrigger)).to.have.length(1);
+    expect(wrapper.find(InfoTooltipWithTrigger)).toHaveLength(1);
   });
   it('sets target="_blank" when openInNewWindow is true', () => {
     props.url = 'https://github.com/apache/incubator-superset';
     wrapper = shallow(factory(props));
-    expect(wrapper.find('a').prop('target')).to.equal(null);
+    expect(wrapper.find('a').prop('target')).toBeNull();
 
     props.openInNewWindow = true;
     wrapper = shallow(factory(props));
-    expect(wrapper.find('a').prop('target')).to.equal('_blank');
+    expect(wrapper.find('a').prop('target')).toBe('_blank');
   });
   it('shows a metric type label when showType is true', () => {
     props.showType = true;
     wrapper = shallow(factory(props));
-    expect(wrapper.find(ColumnTypeLabel)).to.have.length(1);
+    expect(wrapper.find(ColumnTypeLabel)).toHaveLength(1);
   });
 });