emit assembly-output header in error

Port of rust-lang/rust#110851.

Author: tamird

src/runtest.rs

@@ -1756,7 +1756,7 @@ actual:\n\
                 // No extra flags needed.
             }
 
-            Some(_) => self.fatal("unknown 'assembly-output' header"),
+            Some(header) => self.fatal(&format!("unknown 'assembly-output' header: {header}")),
             None => self.fatal("missing 'assembly-output' header"),
         }