tests: Reduce risk of port collision and remove bash dependency.

tests: Reduce risk of port collision and remove bash dependency.

A few tests need a random TCP port on which to listen for connections.
Until now, the tests have used the $RANDOM bash extension to do this, but
this runs the risk of occasionally colliding with an in-use port.  This
commit removes the bash dependency by switching to using a small Perl
program to pick random ports and reduces the risk of collision by
attempting to bind the port that it chooses.

Reported-by: Timothy Chen <tchen@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>