tests: switch default LVM pool to qubes_dom0/vm-pool
tests: switch default LVM pool to qubes_dom0/vm-pool

This is now the installer default. Anyway, it is still possible to
override it with an environment variable.

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