test: add an automatic test for the telnet interface

test: add an automatic test for the telnet interface

Some important things:
 * the test is deactivated by default as it needed netcat. To activate it you
just need to add check_SCRIPTS to the TESTS variable (I won't do it unless we
think that netcat is present on every hosts that run the make check?)
 * the test is really simple for the moment (that's more a POC)
 * we can write a small netcat-like tool to activate this test by default...