internal/testutil: Fix Snake giving false positives for long data

internal/testutil/testutil.go

@@ -57,6 +57,9 @@ func (sn Snake) Check(data []byte) (ok bool, n int) {
 			if !found { return false, n }
 		}
 	}
+	if n < len(data) {
+		return false, n
+	}
 	return true, n
 }