selftests/bpf: Add negative test cases for snprintf

selftests/bpf: Add negative test cases for snprintf

This patch adds a couple negative test cases with a trailing % at the
end of the format string. The %p% case was fixed by the previous commit,
whereas the %s% case was already successfully rejected before.

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/0669bf6eb4f9e5bb10e949d60311c06e2d942447.1751395489.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>