selftests/bpf: Add test for unstable CT lookup API

This tests that we return errors as documented, and also that the kfunc
calls work from both XDP and TC hooks.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220114163953.1455836-8-memxor@gmail.com


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