HPCC-12238 Add a child hashaggregate test

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #29 from jakesmith/hpcc-12238

HPCC-12238 Add a child hashaggregate test

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>