selftests/bpf: add tcx netns cookie tests

Add netns cookie test that verifies the helper is now supported and work
in the context of tc programs.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
Link: https://lore.kernel.org/r/20241007095958.97442-2-mahe.tardy@gmail.com


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