(tests) Implement tests for DNS header parsing

* implemented 3 tests
  * valid opcodes
  * invalid opcodes
  * header too short
* fixed byteorder and bit shifting for flags
* added hexdump for main function for testing purposes

src/main.rs

@@ -32,10 +32,18 @@ fn listen() -> (
     )
 }
 
+fn print_hex(bytes: &[u8]) {
+    let stdout = std::io::stdout();
+    let mut stdout = stdout.lock();
+    let mut hxp = hexyl::Printer::new(&mut stdout, true, hexyl::BorderStyle::None, false);
+    let _ = hxp.print_all(bytes);
+}
+
 fn main() {
     let (thread_udp, _thread_udp_tx, thread_udp_rx) = listen();
 
     for msg in thread_udp_rx.iter() {
+        print_hex(&msg);
         let hdr_struct = DNSHeader::from_udp_datagram(&msg).unwrap();
         dbg!(hdr_struct);
     }