Improve LVM test reliability

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.

Sometimes, creating a volume group would result in a D-Bus timeout.  Those timeout are removed here, but the problem still seems to persist: now the tests time out.  Thus, volume group creation gets stuck sometimes.  Maybe vgcreate is asking questions, but its stdin is /dev/null, so hmm...