Revert "Update dependency ember-qunit to v9 (#1287)"

This reverts commit f417698.

Author: NullVoxPopuli

package.json

@@ -75,7 +75,7 @@
     "ember-cli-inject-live-reload": "2.1.0",
     "ember-data": "5.3.11",
     "ember-load-initializers": "2.1.2",
-    "ember-qunit": "9.0.1",
+    "ember-qunit": "8.1.1",
     "ember-resolver": "11.0.1",
     "ember-source": "5.12.0",
     "ember-source-channel-url": "3.0.0",