Because this was using a set, the order in which property sheets were
used when generating classes was not deterministic.
This can make a different if two property sheets define the same
property. This was the case for short_title property, which was defined
with different default values in DublinCore and SimpleItem property
sheets. With this change the behavior is always same when hash
randomization is enabled.

Property sheets can be defined in classes or on the portal type. The
test only covers the case that a property defined on the portal type
must override the same property defined on a class, but the expected
behavior is also that property sheets defined on a class override
property sheets from parent class.