systests: random_free_port: fix EADDRINUSE flake

Fixes infrequent but annoying flake in which system tests
call random_free_port(), get a nice-looking port, then
fail with "bind: address already in use".

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/system/helpers.network.bash

@@ -251,6 +251,7 @@ function port_is_bound() {
     *"."*)
         grep -e "^[^:]*: $(ipv4_to_procfs "${address}"):${port}"    \
              -e "^[^:]*: $(ipv4_to_procfs "0.0.0.0"):${port}"       \
+             -e "^[^:]*: $(ipv4_to_procfs "127.0.0.1"):${port}"     \
              -q "/proc/net/${proto}"
         ;;
     *)