selftests/bpf: Check VLAN tag and proto in xdp_metadata

Verify, whether VLAN tag and proto are set correctly.

To simulate "stripped" VLAN tag on veth, send test packet from VLAN
interface.

Also, add TO_STR() macro for convenience.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20231205210847.28460-19-larysa.zaremba@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>