fix(showcase): add missing '@ember/test-waiters' dependency

Author: aklkv

showcase/config/ember-try.js

@@ -18,6 +18,7 @@ module.exports = async function () {
         npm: {
           devDependencies: {
             '@ember/test-helpers': '^2.0.0',
+            '@ember/test-waiters': '^3.1.0',
             'ember-a11y-testing': '^5.2.1',
             'ember-cli': '~3.28.0',
             'ember-qunit': '^6.0.0',