Add a tcp loopback tests

Usage
tcploopback server-address client-address

e.g.
tcploopback tcp!127.0.0.1!8 tcp!127.0.0.1!8

only watch the loopback address.

or
tcploopback tcp!*!8 tcp!127.0.0.1!8
All interfaces

Will read from stdin, send to server, print what it returns.
Should work but I had trouble due to other issues, here for your
viewing pleasure.

Signed-off-by: Ronald G. Minnich <rminnich@google.com>