fix test_sendfile_dragonfly/test_recverr tests.

remove as_bytes().len() calls when we can call len() on strings.
```
error: needless call to `as_bytes()`
```

test/sys/test_socket.rs

@@ -2914,7 +2914,7 @@ mod linux_errqueue {
         )
         .unwrap();
         // The sent message / destination associated with the error is returned:
-        assert_eq!(msg.bytes, MESSAGE_CONTENTS.as_bytes().len());
+        assert_eq!(msg.bytes, MESSAGE_CONTENTS.len());
         // recvmsg(2): "The original destination address of the datagram that caused the error is
         // supplied via msg_name;" however, this is not literally true.  E.g., an earlier version
         // of this test used 0.0.0.0 (::0) as the destination address, which was mutated into

test/test_sendfile.rs

@@ -143,7 +143,7 @@ fn test_sendfile_dragonfly() {
         + &trailer_strings.concat();
 
     // Verify the message that was sent
-    assert_eq!(bytes_written as usize, expected_string.as_bytes().len());
+    assert_eq!(bytes_written as usize, expected_string.len());
 
     let mut read_string = String::new();
     let bytes_read = rd.read_to_string(&mut read_string).unwrap();