This ensures that we do not trigger assertions inside HCS by tring to
call hcsshim.DestroyLayer on the parent of a currently-activated layer.
It also deactivates the layers before deletion, to ensure we trigger or
avert file-in-use failures due to leftover state from the tests with
more detail than 'destroy failed'.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>