Fix namespace in integration tests

This commit also removed the redundant directory EmailEditor from integration tests directory.
[MAILPOET-6216]

packages/php/email-editor/tests/integration/Engine/EmailEditorTest.php

@@ -0,0 +1,36 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\EmailEditor\Engine;
+
+class EmailEditorTest extends \MailPoetTest {
+  /** @var EmailEditor */
+  private $emailEditor;
+
+  /** @var callable */
+  private $postRegisterCallback;
+
+  public function _before() {
+    parent::_before();
+    $this->emailEditor = $this->diContainer->get(EmailEditor::class);
+    $this->postRegisterCallback = function ($postTypes) {
+      $postTypes[] = [
+        'name' => 'custom_email_type',
+        'args' => [],
+        'meta' => [],
+      ];
+      return $postTypes;
+    };
+    add_filter('mailpoet_email_editor_post_types', $this->postRegisterCallback);
+    $this->emailEditor->initialize();
+  }
+
+  public function testItRegistersCustomPostTypeAddedViaHook() {
+    $postTypes = get_post_types();
+    $this->assertArrayHasKey('custom_email_type', $postTypes);
+  }
+
+  public function _after() {
+    parent::_after();
+    remove_filter('mailpoet_email_editor_post_types', $this->postRegisterCallback);
+  }
+}