selftests/bpf: Add a kernel flag test for LSM bpf hook

selftests/bpf: Add a kernel flag test for LSM bpf hook

This test exercises the kernel flag added to security_bpf by
effectively blocking light-skeletons from loading while allowing
normal skeletons to function as-is. Since this should work with any
arbitrary BPF program, an existing program from LSKELS_EXTRA was
used as a test payload.

Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250310221737.821889-3-bboscaccy@linux.microsoft.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>