Resolve #3041318 "Various random fails"

Closes #3041318

core/modules/field_ui/tests/src/Functional/ManageFieldsLifecycleTest.php

@@ -292,7 +292,7 @@ protected function addPersistentFieldStorage(): void {
     $this->assertSession()->pageTextContains('Re-use an existing field');
 
     // Ensure that we test with a label that contains HTML.
-    $label = $this->randomString(4) . '<br/>' . $this->randomString(4);
+    $label = $this->randomMachineName(4) . '<br/>' . $this->randomMachineName(4);
     // Add a new field for the orphaned storage.
     $this->fieldUIAddExistingField("admin/structure/types/manage/page", $this->fieldName, $label);
   }