selftests/bpf: test_xdp_veth: Add XDP flags to prog_configuration

XDP flags are hardcoded to 0 at attachment.

Add flags attributes to the struct prog_configuration to allow flag
modifications for each test case.

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-v4-8-970b33678512@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>