selftests/bpf: don't hardcode iptables/nc path in test_tcpnotify_user

system() is calling shell which should find the appropriate full path
via $PATH. On some systems, full path to iptables and/or nc might be
different that we one we have hardcoded.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>