fix

Author: Jiayu Liu

datafusion/src/physical_plan/windows.rs

@@ -562,7 +562,7 @@ mod tests {
                 "WindowAggExec requires a single input partition".to_owned()
             );
         } else {
-            assert!(false, "Expect an internal error to happen");
+            unreachable!("Expect an internal error to happen");
         }
         Ok(())
     }