compiletest: emit assembly-output header in error

Author: tamird

src/tools/compiletest/src/runtest.rs

@@ -2332,7 +2332,7 @@ impl<'test> TestCx<'test> {
                 // 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"),
         }