selftests/bpf: Add tests for iter arg check

Add selftests to cover argument type check for iterator kfuncs, and
cover all three kinds (new, next, destroy). Without the fix in the
previous patch, the selftest would not cause a verifier error.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241203000238.3602922-3-memxor@gmail.com


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