[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/models/Plugin_spec.js

@@ -1,35 +1,39 @@
-import { describe, it } from 'mocha';
-import { expect } from 'chai';
 import Plugin from '../../../../src/visualizations/core/models/Plugin';
 
 describe('Plugin', () => {
   it('exists', () => {
-    expect(Plugin).to.not.equal(undefined);
+    expect(Plugin).toBeDefined();
   });
 
   describe('new Plugin()', () => {
     it('creates a new plugin', () => {
       const plugin = new Plugin();
-      expect(plugin).to.be.instanceof(Plugin);
+      expect(plugin).toBeInstanceOf(Plugin);
     });
   });
 
   describe('.configure(config, replace)', () => {
-    it('extends the default config with given config when replace is not set or false', () => {
-      const plugin = new Plugin();
-      plugin.configure({ key: 'abc', foo: 'bar' });
-      plugin.configure({ key: 'def' });
-      expect(plugin.config).to.deep.equal({ key: 'def', foo: 'bar' });
-    });
-    it('replaces the default config with given config when replace is true', () => {
-      const plugin = new Plugin();
-      plugin.configure({ key: 'abc', foo: 'bar' });
-      plugin.configure({ key: 'def' }, true);
-      expect(plugin.config).to.deep.equal({ key: 'def' });
-    });
+    it(
+      'extends the default config with given config when replace is not set or false',
+      () => {
+        const plugin = new Plugin();
+        plugin.configure({ key: 'abc', foo: 'bar' });
+        plugin.configure({ key: 'def' });
+        expect(plugin.config).toEqual({ key: 'def', foo: 'bar' });
+      },
+    );
+    it(
+      'replaces the default config with given config when replace is true',
+      () => {
+        const plugin = new Plugin();
+        plugin.configure({ key: 'abc', foo: 'bar' });
+        plugin.configure({ key: 'def' }, true);
+        expect(plugin.config).toEqual({ key: 'def' });
+      },
+    );
     it('returns the plugin itself', () => {
       const plugin = new Plugin();
-      expect(plugin.configure({ key: 'abc' })).to.equal(plugin);
+      expect(plugin.configure({ key: 'abc' })).toBe(plugin);
     });
   });
 
@@ -38,11 +42,11 @@ describe('Plugin', () => {
       const plugin = new Plugin();
       plugin.configure({ key: 'abc', foo: 'bar' });
       plugin.resetConfig();
-      expect(plugin.config).to.deep.equal({});
+      expect(plugin.config).toEqual({});
     });
     it('returns the plugin itself', () => {
       const plugin = new Plugin();
-      expect(plugin.resetConfig()).to.equal(plugin);
+      expect(plugin.resetConfig()).toBe(plugin);
     });
   });
 });