Bind to random port numbers in unit tests

Bind to random port numbers in unit tests

Some unit tests need various managers to be created before they are run.
The interface manager spawned during libns tests listens on a fixed port
number, which causes intermittent issues when multiple tests using an
interface manager are run concurrently.  Make the interface manager
listen on a randomized port number to greatly reduce the risk of
multiple unit tests using the same port concurrently.