[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/reducers/sliceEntities_spec.js

@@ -1,5 +1,3 @@
-import { expect } from 'chai';
-
 import {
   FETCH_ALL_SLICES_FAILED,
   FETCH_ALL_SLICES_STARTED,
@@ -10,7 +8,7 @@ import sliceEntitiesReducer from '../../../../src/dashboard/reducers/sliceEntiti
 
 describe('sliceEntities reducer', () => {
   it('should return initial state', () => {
-    expect(sliceEntitiesReducer({}, {})).to.deep.equal({});
+    expect(sliceEntitiesReducer({}, {})).toEqual({});
   });
 
   it('should set loading when fetching slices', () => {
@@ -19,7 +17,7 @@ describe('sliceEntities reducer', () => {
         { isLoading: false },
         { type: FETCH_ALL_SLICES_STARTED },
       ).isLoading,
-    ).to.equal(true);
+    ).toBe(true);
   });
 
   it('should set slices', () => {
@@ -28,12 +26,12 @@ describe('sliceEntities reducer', () => {
       { type: SET_ALL_SLICES, slices: { 1: {}, 2: {} } },
     );
 
-    expect(result.slices).to.deep.equal({
+    expect(result.slices).toEqual({
       1: {},
       2: {},
       a: {},
     });
-    expect(result.isLoading).to.equal(false);
+    expect(result.isLoading).toBe(false);
   });
 
   it('should set an error on error', () => {
@@ -44,7 +42,7 @@ describe('sliceEntities reducer', () => {
         error: { responseJSON: { message: 'errorrr' } },
       },
     );
-    expect(result.isLoading).to.equal(false);
-    expect(result.errorMessage.indexOf('errorrr')).to.be.above(-1);
+    expect(result.isLoading).toBe(false);
+    expect(result.errorMessage.indexOf('errorrr')).toBeGreaterThan(-1);
   });
 });