Create renderer for form headers

[MAILPOET-2613]

tests/unit/Form/Block/HeadingTest.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace MailPoet\Test\Form\Block;
+
+use MailPoet\Form\Block\Heading;
+
+class HeadingTest extends \MailPoetUnitTest {
+  /** @var Heading */
+  private $heading;
+
+  public function _before() {
+    parent::_before();
+    $this->heading = new Heading();
+  }
+
+  public function testItShouldRenderHeading() {
+    $html = $this->heading->render([], []);
+    expect($html)->startsWith('<h2');
+  }
+}