selftests/bpf: add tests for bpf_ct_set_nat_info kfunc

Introduce self-tests for bpf_ct_set_nat_info kfunc used to set the
source or destination nat addresses/ports.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/803e33294e247744d466943105879414344d3235.1663778601.git.lorenzo@kernel.org


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