selftests/bpf: Integrate test_xdp_veth into test_progs

test_xdp_veth.sh tests that XDP return codes work as expected, by bringing
up multiple veth pairs isolated in different namespaces, attaching specific
xdp programs to each interface, and ensuring that the whole chain allows to
ping one end interface from the first one. The test runs well but is
currently not integrated in test_progs, which prevents it from being run
automatically in the CI infrastructure.

Rewrite it as a C test relying on libbpf to allow running it in the CI
infrastructure. The new code brings up the same network infrastructure and
reuses the same eBPF programs as test_xdp_veth.sh, for which skeletons are
already generated by the bpf tests makefile.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/bpf/20240721-convert_test_xdp_veth-v4-2-23bdba21b2f9@bootlin.com


Signed-off-by: Andrii Nakryiko <andrii@kernel.org>