Specify the component should call the save action

We reproduce the acceptance test error at the component level:

Inner Red: Expected submit handler to be called

Our code doesn't refer to this submitHandler attribute yet, so it's never called.

tests/integration/components/post-form-test.js

@@ -1,6 +1,6 @@
 import { module, test } from 'qunit';
 import { setupRenderingTest } from 'ember-qunit';
-import { render } from '@ember/test-helpers';
+import { render, click } from '@ember/test-helpers';
 import hbs from 'htmlbars-inline-precompile';
 
 module('Integration | Component | post-form', function(hooks) {
@@ -12,4 +12,18 @@ module('Integration | Component | post-form', function(hooks) {
     assert.dom('.js-post-form-title').exists();
     assert.dom('.js-post-form-body').exists();
   });
+
+  test('it calls the submit handler', async function(assert) {
+    let submitHandlerCalled = false;
+    this.set('testSubmitHandler', () => {
+      submitHandlerCalled = true;
+    });
+
+    await render(hbs`{{post-form submitHandler=(action testSubmitHandler)}}`);
+
+    await click('.js-post-form-save');
+
+    assert.ok(submitHandlerCalled,
+          "Expected submit handler to be called");
+  });
 });