selftests/bpf: Add TX side to xdp_metadata

Request TX timestamp and make sure it's not empty.
Request TX checksum offload (SW-only) and make sure it's resolved
to the correct one.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231127190319.1190813-12-sdf@google.com


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