chore: Temporarily downgraded @ember/test-helpers to v2.7.0 so that 3.28 and 4.4 ember-try scenarios run

Author: ijlee2

test-app/config/ember-try.js

@@ -11,6 +11,10 @@ module.exports = async function () {
         name: 'ember-lts-3.28',
         npm: {
           devDependencies: {
+            '@ember/test-helpers': '2.7.0',
+            '@types/ember__test-helpers': '2.8.3',
+            '@types/ember-qunit': '6.1.1',
+            'ember-qunit': '6.0.0',
             'ember-source': '~3.28.0',
           },
         },
@@ -19,6 +23,10 @@ module.exports = async function () {
         name: 'ember-lts-4.4',
         npm: {
           devDependencies: {
+            '@ember/test-helpers': '2.7.0',
+            '@types/ember__test-helpers': '2.8.3',
+            '@types/ember-qunit': '6.1.1',
+            'ember-qunit': '6.0.0',
             'ember-source': '~4.4.0',
           },
         },