sql: Clean up builtin function testing of arrays
sql: Clean up builtin function testing of arrays

This change:
- Addresses a TODO about array type assertions
- Adds testing for multidimensional arrays
- Moves testing of `current_schema` to a more appropriate place

paperstreet/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,maxlang/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach