chore(test-app): import ember-data string transform

Author: BobrImperator

packages/classic-test-app/app/transforms/string.js

@@ -0,0 +1 @@
+export { StringTransform as default } from '@ember-data/serializer/transform';

packages/test-app/app/transforms/string.js

@@ -0,0 +1 @@
+export { StringTransform as default } from '@ember-data/serializer/transform';

packages/test-esa/config/ember-try.js

@@ -20,7 +20,7 @@ module.exports = function () {
           npm: {
             devDependencies: {
               'ember-cli': '~3.28.0',
-              'ember-data': '~3.28.0',
+              'ember-data': '~4.4.0',
               'ember-source': '~3.28.0',
               'ember-qunit': '~6.0.0',
               '@ember/test-helpers': '~2.7.0',