sql: Fix flake in TestQueryCounts
sql: Fix flake in TestQueryCounts

It was conceivable that the system migration that does a DDL statement
could be retried, causing the DDL count to be 2 instead of 1 in some
cases. This fixes that and is also less fragile to future migrations
being added.

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