selftests/bpf: Add negative tests for new nf_conntrack kfuncs

Test cases we care about and ensure improper usage is caught and
rejected by the verifier.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220721134245.2450-13-memxor@gmail.com


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