Test coverage for Item clones - Item_cache*

The idea is that we need test our deep clones for our various Item types. It is hard, or even impossible to create testcases that create/use deep Item clones today. But Parallel Query does use item clones extensively.

One of the places that deep clones are created is key part generation for lookups into Split-Materialized tables.

We will be targeting Item_cache* classes in this MDEV (The next step will be to cover aggregate (Item_sum *) items)

The idea is:

• use a query shape that creates clones (Split-Materialized)
• Inside a DBUG_EXECUTE_IF(), replace the original item with a clone
  • Also run a debug check that the clone is indeed a proper deep clone.
• Continue running the query to make the clone is fully usable.