Update dependency ember-a11y-testing to v4

Author: renovate-bot

package.json

@@ -60,7 +60,7 @@
     "babel-eslint": "10.1.0",
     "babel-plugin-transform-object-rest-spread": "6.26.0",
     "broccoli-asset-rev": "3.0.0",
-    "ember-a11y-testing": "3.0.2",
+    "ember-a11y-testing": "4.0.0",
     "ember-api-actions": "0.2.9",
     "ember-auto-import": "1.10.1",
     "ember-cli": "3.24.0",