tests: Add 5 seconds sleep after snapshot revert
tests: Add 5 seconds sleep after snapshot revert

Time it takes to revert VMs to snapshots varies a lot so lets wait at
least 5 seconds before checking for the process to have finished.

https://bugzilla.gnome.org/show_bug.cgi?id=744703

vbenes/gnome-boxes,vbenes/gnome-boxes