Update dependency @ember/test-helpers to v2.9.0 (#613)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -31,7 +31,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "2.0.0",
-    "@ember/test-helpers": "2.8.1",
+    "@ember/test-helpers": "2.9.0",
     "@glimmer/syntax": "0.84.2",
     "@glimmer/syntax-0.35.11": "npm:@glimmer/syntax@0.35.11",
     "@glimmer/syntax-0.37.1": "npm:@glimmer/syntax@0.37.1",