selftests/harness: Move test child waiting logic

In order to better handle timeout failures, rearrange the child waiting
logic into a separate function. This is mostly a copy/paste with an
indentation change. To handle pid tracking, a new field is added for
the child pid. Also move the alarm() pairing into the function.

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