Fix a subtle bug in the CanTieredAlloc test. The property we were
testing for was actually not valid with the addition of an ipolicy.
The lower policy bound for disk was 4 times the unit for disks (256M).
So, a valid node have free disk only slightly larger than the ipolicy
lower bound. When shrinking disks, however, we go in steps of full
units---and thus might miss the window of opportunity if that is smaller
that a full unit. So, only use nodes that have at least 5 units worth
of resources for every resource.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>