only in patch2:
unchanged:
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -940,6 +940,9 @@ protected function setUp() {
       $this->rebuildContainer();
     }
 
+    // Place messages block so its visible for tests.
+    $this->drupalPlaceBlock('system_page_messages_block');
+
     // Reset/rebuild all data structures after enabling the modules, primarily
     // to synchronize all data structures and caches between the test runner and
     // the child site.