selftests/bpf: Add test for reading from STACK_INVALID slots

Ensure that when CAP_PERFMON is dropped, and the verifier sees
allow_ptr_leaks as false, we are not permitted to read from a
STACK_INVALID slot. Without the fix, the test will report unexpected
success in loading.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204044757.1483141-5-memxor@gmail.com


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