test(common): add missing assertion in full_rewind test (#3926)

Co-authored-by: 1911860538 <[email protected]>

Author: 1911860538

src/common/io/rewind.rs

@@ -156,5 +156,7 @@ mod tests {
 
         let mut buf = [0; 5];
         stream.read_exact(&mut buf).await.expect("read1");
+
+        assert_eq!(&buf, &underlying);
     }
 }