Widen @ember/test-helpers peer dependency declaration

v4 of `@ember/test-helpers` was released lately, and appears to work just fine with `ember-a11y-testing` since the breaking changes in that release are irrelevant to this addon :)

Author: Turbo87

package.json

@@ -131,7 +131,7 @@
     "webpack": "^5.65.0"
   },
   "peerDependencies": {
-    "@ember/test-helpers": "^3.0.3",
+    "@ember/test-helpers": "^3.0.3 || ^4.0.2",
     "qunit": ">= 2"
   },
   "peerDependenciesMeta": {