Utilize jest-environment-jsdom-sixteen for tests

This allows us to utilize the newest version of jsdom (16) in jest. Our
current version of jest relies on version 11. The newer version of
jsdom brings a lot of features, e.g. Custom Elements which we need for
gl_emoji tests.

package.json

@@ -155,6 +155,7 @@
     "xterm": "^3.5.0"
   },
   "devDependencies": {
+    "acorn": "^6.3.0",
     "@babel/plugin-transform-modules-commonjs": "^7.10.1",
     "@gitlab/eslint-plugin": "3.1.0",
     "@vue/test-utils": "^1.0.0-beta.30",
@@ -183,7 +184,7 @@
     "jasmine-jquery": "^2.1.1",
     "jest": "^24.1.0",
     "jest-canvas-mock": "^2.1.2",
-    "jest-environment-jsdom": "^24.0.0",
+    "jest-environment-jsdom-sixteen": "^1.0.0",
     "jest-junit": "^6.3.0",
     "jest-util": "^24.0.0",
     "jsdoc": "^3.5.5",

spec/frontend/environment.js

@@ -2,7 +2,7 @@
 
 const path = require('path');
 const { ErrorWithStack } = require('jest-util');
-const JSDOMEnvironment = require('jest-environment-jsdom');
+const JSDOMEnvironment = require('jest-environment-jsdom-sixteen');
 
 const ROOT_PATH = path.resolve(__dirname, '../..');