HPCC-12041 Add some group sort tests

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-12041 Make the groupsings local and add another stress test

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #27 from ghalliday/issue12041

HPCC-12041 Add some group sort tests

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>