selftests/bpf: add more test cases for check_cfg()

Add a few more simple cases to validate proper privileged vs unprivileged
loop detection behavior. conditional_loop2 is the one reported by Hao
Sun that triggered this set of fixes.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Suggested-by: Hao Sun <sunhao.th@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231110061412.2995786-2-andrii@kernel.org


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