Fix KqpOlapAggregations.Aggregation_ResultCountAll_Filter

[zverevgeny]

test in currently disabled in CI

ydb/.github/config/muted_ya.txt

Line 13 in a169e9b

ydb/core/kqp/ut/olap KqpOlapAggregations.Aggregation_ResultCountAll_FilterL

[fexolm]

Basically, this test was failing because check for resultCount is incorrect. This check is done very early before reducing aggregation result. It expects that for every partition aggregation would return single row or nothing, but it is not always true. Sometimes partitions are spitted to several record batches, and in this case at this stage resulting batch would contain a row for each input batch.

@ivanmorozov333 told that 3 should be a good threshold for that test. If this check would fail again, this could be an indicator that something could be wrong.