selftests/seccomp: Report event mismatches more clearly

When running under tracer, more explicitly report the status and event
mismatches to help with debugging. Additionally add an "immediate kill"
test when under tracing to verify that fatal SIGSYS behaves the same
under ptrace or seccomp tracing.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/20211103163039.2104830-3-keescook@chromium.org