(maint) Fix acceptance test for scheduled_task deletion

This test was accidentally checking that the scheduled task was still in
place in the Windows scheduled task list. Because the
Win32::TaskScheduler#delete call seems to be asynchronous, as long as
the test for scheduled task happened fast enough, it was still seen as
present and the test 'passed'.  In fact, puppet resource was scheduling
the task to be removed, and it does get removed.  This test began to fail
when the vms slowed down enough that the broken verification step began
to fail because it was no longer executing fast enough to see the
transient task prior to its deletion completing.

This change corrects the verification step to verify by asserting that
the schtasks.exe list command failed (did not show the deleted task).
And it wraps it in a timeout to allow it a few seconds for the scheduled
tasks state to finalize.