Tests: Wait for disks to be free before using them for LVM.

There was a race between check-storage and storaged where we would try
to use a disk for a new volume group before storaged has noticed that
it is no longer used for the old volume group. In that case, that disk
would not be offered in the dialog, and the test would fail.

Closes #241

Reviewed-by: Stef Walter <stefw@redhat.com>