fix(vrt): switch to Jest DOM-only testing (#3410)

Author: amyleesalesforce

jest.helpers.js

@@ -7,6 +7,7 @@
 import { assertMatchesDOM } from '@salesforce-ux/instant-vrt/matcher';
 import fetch from 'isomorphic-fetch';
 import ReactDOM from 'react-dom/server';
+import renderer from 'react-test-renderer';
 
 import { beautify } from './shared/utils/beautify';
 import { renderWithBetterError } from './shared/utils/react';
@@ -21,6 +22,11 @@ export default (directory, port) => {
   });
 
   return {
+    matchesMarkup: reactElement => {
+      // https://jestjs.io/docs/en/snapshot-testing#snapshot-testing-with-jest
+      const dom = renderer.create(reactElement).toJSON();
+      expect(dom).toMatchSnapshot();
+    },
     matchesMarkupAndStyle: async element => {
       const renderedMarkup =
         typeof element === 'string'

package.json

@@ -131,6 +131,7 @@
     "raw-loader": "0.5.1",
     "react": "16.2.0",
     "react-dom": "16.2.0",
+    "react-test-renderer": "16.4.1",
     "rimraf": "2.6.1",
     "sass-loader": "6.0.6",
     "strip-indent": "2.0.0",