[unittests] Derive Occupied from Unused when given.

When both, OccupiedAndKnown and Unused are given, use the former only
for the Known values. The relation Unused \union Occupied must always
hold.

This allows us to specify Known independently of Occupied. It is needed
for an artificial test case in https://reviews.llvm.org/D32025.

llvm-svn: 301284