tests: wait for DispVM's qubes.VMShell exit
tests: wait for DispVM's qubes.VMShell exit

It isn't enough to wait for window to disappear, the service may still
be running. And if it is, test cleanup logic will complain about FD
leak.
To avoid deadlock on some test failure, do it with a timeout.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin