Replace manual mock/jsdom with webextensions-jsdom

package.json

@@ -16,7 +16,6 @@
     "eslint-plugin-no-unsanitized": "^2.0.0",
     "eslint-plugin-promise": "^3.4.0",
     "htmllint-cli": "0.0.7",
-    "jsdom": "^11.6.2",
     "json": "^9.0.6",
     "mocha": "^6.2.2",
     "npm-run-all": "^4.0.0",
@@ -25,7 +24,8 @@
     "stylelint": "^7.9.0",
     "stylelint-config-standard": "^16.0.0",
     "stylelint-order": "^0.3.0",
-    "web-ext": "^2.9.3"
+    "web-ext": "^2.9.3",
+    "webextensions-jsdom": "^1.1.0"
   },
   "homepage": "https://github.com/mozilla/multi-account-containers#readme",
   "license": "MPL-2.0",