selftests/bpf: test_xdp_veth: Add XDP broadcast redirection tests

selftests/bpf: test_xdp_veth: Add XDP broadcast redirection tests

XDP redirections with BPF_F_BROADCAST and BPF_F_EXCLUDE_INGRESS flags
are tested by test_xdp_redirect_multi.sh but not within the test_progs
framework.

Add a broadcast test case in test_xdp_veth.c to test them.
Use the same BPF programs than the one used by
test_xdp_redirect_multi.sh.
Use a BPF map to select the broadcast flags.
Use a BPF map with an entry per veth to check whether packets are
received or not

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/20250212-redirect-multi-v5-4-fd0d39fca6e6@bootlin.com