Alternate, and portable, stubbing of execution for upstart spec.

Previously this spec used to execute an external command in order to set the
Ruby $? global to a Process::Status object with a successful exit. This
was fine, but not portable to Windows, and not exactly efficient.

Instead, we now stub the exitcode on the $? object, whatever it is, to report
successful exit, and carry on. That works, stubbing the instance, to make
this both fairly robust and entirely portable.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>