selftests/bpf: add more cases for __arg_trusted __arg_nullable args

selftests/bpf: add more cases for __arg_trusted __arg_nullable args

Add extra layer of global functions to ensure that passing around
(trusted) PTR_TO_BTF_ID_OR_NULL registers works as expected. We also
extend trusted_task_arg_nullable subtest to check three possible valid
argumements: known NULL, known non-NULL, and maybe NULL cases.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240202190529.2374377-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>