selftests/bpf: Monitor traffic for sockmap_listen.

Enable traffic monitor for each subtest of sockmap_listen.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240815053254.470944-6-thinker.li@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>