Add test for the functionality acquired by the glimmer upgrade

packages/ember/tests/integration/glimmer-features/functions-as-helpers.js

@@ -0,0 +1,25 @@
+import {moduleFor, RenderingTestCase, runkTask} from 'internal-test-helpers';
+import { Component } from '@ember/-internals/glimmer';
+
+
+// Initial implementation: https://github.com/glimmerjs/glimmer-vm/pull/1348
+moduleFor('Glimmer Features - functions as helpers', class extends RenderingTestCase {
+  async ['@test plain functions work as helpers'](assert) {
+    let count = 0;
+    this.addComponent('demo', {
+      template: `{{ (this.hello) }}`,
+      ComponentClass: class Demo extends Component {
+        hello = () => {
+          count++;
+          return 'plain function';
+        }
+      },
+    });
+
+    assert.equal(count, 0);
+
+    this.render('<Demo />');
+
+    assert.equal(count, 1);
+  }
+})