ENG-5601: Fix new findBlock test in EE.

Actually allocate some memory to fake tuple blocks, because the tuple block
pointers will try to free them afterwards.
ENG-5601: Fix tuple block cleanup in the COW iterator.

The COW iterator was releasing a tuple block once it starts working on it. This
tricked the COW context to think that a tuple falling into the block is not
worth dirtying even though it is.

Now the COW iterator just erases blocks from the tuple block map when it
finishes working on them.