test(query-engine): no test for a SQL quantile roll-up #871

rollup_tests.rs covers SQL roll-ups only for HLL (COUNT(DISTINCT)), over one window. --planner saved now writes SQL quantile roll-ups too: sketch-bench's p99 CUBE reads a DDSketch grouped by the finest grouping, over a lookback of several tumbling windows. Nothing tests that shape through handle_query_sql.