tests: improve shutdown timeout handling
tests: improve shutdown timeout handling

Instead of waiting 1sec, wait up to 5sec but skip when vm is shut off.
This fix tests on slow machines, including openQA nested virt.

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