selftests/harness: fix spelling mistake "SIGARLM" -> "SIGALRM"

There a few identical spelling mistakes, fix these.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>