Test tst-poll.cc can not be runned twice

Test tst-poll throws an exception on second execution:
../tst-poll.cc(78): fatal error in "test_polling_on_one_socket":
critical check bind(s, (struct sockaddr *) &laddr, sizeof(laddr)) == 0
failed. Test test_polling_on_one_socket left the binded socket open -
cuasing the second execution to fail on bind with EADDRINUSE (98)
/* Address already in use */

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Shlomi Livne <shlomi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>