selftests/bpf: Add verifier tests for trusted kfunc args

Make sure verifier rejects the bad cases and ensure the good case keeps
working. The selftests make use of the bpf_kfunc_call_test_ref kfunc
added in the previous patch only for verification.

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220721134245.2450-11-memxor@gmail.com


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