selftests/bpf: test_xdp_veth: Add prog_config[] table

selftests/bpf: test_xdp_veth: Add prog_config[] table

The BPF program attached to each veth is hardcoded through the
use of the struct skeletons. It prevents from re-using the initialization
code in new test cases.

Replace the struct skeletons by a bpf_object table.
Add a struct prog_configuration that holds the name of BPF program to
load on a given veth pair.
Use bpf_object__find_program_by_name() / bpf_xdp_attach() API instead of
bpf_program__attach_xdp() to retrieve the BPF programs from their names.
Detach BPF progs in the cleanup() as it's not automatically done by this
API.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250131-redirect-multi-v4-7-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>