MissingObserver polyfill for jest environment

References by noVNC (@novnc/nvnc/core/util/events.js).
Workaround just for the `jest` testing - the MutationObserver
is available in browsers.

Use of `browser` jest configuration option does not help.

package.json

@@ -79,6 +79,7 @@
     "istanbul-reports": "1.1.4",
     "jest": "^22.4.2",
     "jest-cli": "^22.4.2",
+    "mutation-observer": "^1.0.3",
     "node-sass": "^4.7.2",
     "prettier": "^1.9.2",
     "prettier-eslint": "^8.8.1",

src/test.env.js

@@ -2,4 +2,10 @@ import 'raf/polyfill';
 import { configure } from 'enzyme';
 import Adapter from 'enzyme-adapter-react-16';
 
+const MutationObserverPolyfill = require('mutation-observer');
+
+// referenced from '@novnc/nvnc/core/util/events.js'
+// The MutationObserver is available in supported browsers, this is workaround for "jest"
+global.MutationObserver = global.MutationObserver || MutationObserverPolyfill;
+
 configure({ adapter: new Adapter() });