Fix flaky spill stats check in aggregation test

The flaky is due to spill check condition is fragile which is based on whether we have received
more than one input at partial aggregation. We shall change to final aggregation and given we
have four drivers so it is not guarantee one aggregation has received more than one input row.
Since we have recorded the spill injection count, then we just rely on this to check spill stats.

velox/functions/lib/aggregates/tests/utils/AggregationTestBase.cpp

@@ -415,7 +415,7 @@ void AggregationTestBase::testAggregationsWithCompanion(
         toPlanStats(task->taskStats()).at(partialNodeId).inputRows;
     const auto finalInputRows =
         toPlanStats(task->taskStats()).at(finalNodeId).inputRows;
-    if (partialInputRows > 1) {
+    if (exec::injectedSpillCount() > 0) {
       EXPECT_LT(0, spilledBytes(*task))
           << "partial inputRows: " << partialInputRows
           << " final inputRows: " << finalInputRows
@@ -854,7 +854,7 @@ void AggregationTestBase::testAggregationsImpl(
         toPlanStats(task->taskStats()).at(partialNodeId).inputRows;
     const auto finalInputRows =
         toPlanStats(task->taskStats()).at(finalNodeId).inputRows;
-    if (partialInputRows > 1) {
+    if (exec::injectedSpillCount() > 0) {
       EXPECT_LT(0, spilledBytes(*task))
           << "partial inputRows: " << partialInputRows
           << " final inputRows: " << finalInputRows