selftests/bpf: Add test for BPF_NEG alu on CONST_PTR_TO_MAP

Add a test case for BPF_NEG operation on CONST_PTR_TO_MAP. Tests if
BPF_NEG operation on map_ptr is rejected in unprivileged mode and is a
scalar value and do not trigger Oops in privileged mode.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Signed-off-by: Brahmajit Das <listout@listout.xyz>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251001191739.2323644-3-listout@listout.xyz


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