selftests/bpf: Migrate test_xdp_vlan.sh into test_progs

test_xdp_vlan.sh isn't used by the BPF CI.

Migrate test_xdp_vlan.sh in prog_tests/xdp_vlan.c.
It uses the same BPF programs located in progs/test_xdp_vlan.c and the
same network topology.
Remove test_xdp_vlan*.sh and their Makefile entries.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250221-xdp_vlan-v1-2-7d29847169af@bootlin.com/