reverted:
--- b/core/modules/simpletest/src/WebTestBase.php
+++ a/core/modules/simpletest/src/WebTestBase.php
@@ -940,9 +940,6 @@
       $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.
diff -u b/core/modules/system/templates/page.html.twig b/core/modules/system/templates/page.html.twig
--- b/core/modules/system/templates/page.html.twig
+++ b/core/modules/system/templates/page.html.twig
@@ -98,6 +98,7 @@
   {{ page.breadcrumb }}
 
   {{ page.messages }}
+  {{ messages }}
 
   {{ page.help }}