Add example module with a deprecation

web/modules/custom/example/tests/src/Functional/DisplayMessageTest.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\Tests\example\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+class DisplayMessageTest extends BrowserTestBase {
+
+  protected $defaultTheme = 'stable';
+
+  protected static $modules = ['filter', 'node', 'example'];
+
+  /** @test */
+  public function a_message_is_displayed_on_nodes(): void {
+    $this->drupalCreateContentType(['type' => 'page']);
+
+    $this->drupalCreateNode();
+
+    $this->drupalGet('node/1');
+
+    $this->assertText('This is a node.');
+  }
+
+}