diff -u b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
--- b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
+++ b/core/tests/Drupal/Tests/PhpUnitWarningsTest.php
@@ -20,8 +20,8 @@
    * @expectedDeprecation The optional $ignoreCase parameter of assertContains() is deprecated and will be removed in PHPUnit 9.
    */
   public function testAssertContains() {
-    $this->assertContains('string', 'aaaastringaaa');
-    $this->assertContains('STRING', 'aaaastringaaa', '', TRUE);
+    $this->assertContains('string', 'aaaa_string_aaa');
+    $this->assertContains('STRING', 'aaaa_string_aaa', '', TRUE);
   }
 
   /**