selftests/harness: Add 30 second timeout per test

In order to keep tests from hanging forever, this adds an alarm signal
to each test run. This assumes an individual test doesn't take longer
than 30 seconds.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>