selftests/seccomp: Test PTRACE_O_SUSPEND_SECCOMP without CAP_SYS_ADMIN

Add a test to check that PTRACE_O_SUSPEND_SECCOMP can't be set without
CAP_SYS_ADMIN through PTRACE_SEIZE or PTRACE_SETOPTIONS.

Signed-off-by: Jann Horn <jannh@google.com>
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>