tools/testing/selftests/bpf/test_tc_tunnel.sh: Fix wait for server bind

Commit f803bcf9208a ("selftests/bpf: Prevent client connect before
server bind in test_tc_tunnel.sh") added code that waits for the
netcat server to start before the netcat client attempts to connect to
it. However, not all calls to 'server_listen' were guarded.

This patch adds the existing 'wait_for_port' guard after the remaining
call to 'server_listen'.

Fixes: f803bcf9208a ("selftests/bpf: Prevent client connect before server bind in test_tc_tunnel.sh")
Signed-off-by: Marco Leogrande <leogrande@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20241202204530.1143448-1-leogrande@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>