Resolve #3505289 "Fix phpstan warnings reported by CI."

Closes #3505289

tests/src/Functional/CreatePageContentTest.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\acquia_optimize\Functional;
 
+use Drupal\Tests\user\Traits\UserCreationTrait;
 use weitzman\DrupalTestTraits\ExistingSiteBase;
 
 /**
@@ -11,6 +12,8 @@ use weitzman\DrupalTestTraits\ExistingSiteBase;
  */
 class CreatePageContentTest extends ExistingSiteBase {
 
+  use UserCreationTrait;
+
   /**
    * A user with permissions to view Optimize button.
    *