selftests/bpf: test_xdp_veth: Add new test cases for XDP flags

The XDP redirection is tested without any flag provided to the
xdp_attach() function.

Add two subtests that check the correct behaviour with
XDP_FLAGS_{DRV/SKB}_MODE flags

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-v4-10-970b33678512@bootlin.com


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