Test suite: wake up the screen from DPMS suspend immediately after restoring a snapshot

I've seen instances of "Display output is not active" where
the screen had blanked immediately after restoring the snapshot,
after `@screen.wake_up` (which moves the mouse and presses a key),
so `xset dpms force on` has not been run: we failed to find
DesktopTailsDocumentation.png. Interestingly this happened
*before* we sync'ed the clock from the host.