java/net/DatagramSocket/DatagramSocketMulticasting.java should be hardened

The java/net/DatagramSocket/DatagramSocketMulticasting.java is vulnerable to receiving datagrams sent by other tests running concurrently on the machine. In particular the method `testSendReceive` should use a body text unique for the method invocation, and ignore any received datagram that do not contain the same body.

The test could also benefit from being refactored to using TestNG or Jupiter rather than using its own assertXxx methods (which would allow to use assertEquals rather than plain assertTrue