Currently, only the $lookup tests in `search_sharded_union_with_lookup.js` use $$SEARCH_META.  We should extend the tests to include $$SEARCH_META in $unionWith tests for all combinations of outer and inner collections sharded or not.

Here's the current state:

https://docs.google.com/spreadsheets/d/1y5Kkn_Z7FVdpgy-u3OsUNjtKSXSrmJnifJAjUMxREpA/edit#gid=0