tests: bufferpool: fix wrong assumptions about pointers and object lifecycles

tests: bufferpool: fix wrong assumptions about pointers and object lifecycles

The test assumed that if a buffer has the same pointer address as
before it is in fact the same mini object and has been re-used by
the pool. This seems to be mostly true, but not always. The buffer
might be destroyed and when a new buffer is created the allocator
might return the same memory that we just freed.

Instead attach a qdata with destroy notify function to buffer
instances we want to track to make sure the buffer actually
gets finalized rather than resurrected and put back into the pool.