HPCC-12200 Roxie splitters aren't very fair

Added initial version of unit test code for roxie splitters to aid in
investigation/testing of this issue

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-12200 Roxie splitters aren't very fair

Extended unit testing code.

Fixed race condition that could lead to rows being left behind in splitter
after no-one wanted them.

Slight optimization of common case of 2-way splitter

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-12200 Roxie splitters aren't very fair

Add more testing

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6606 from richardkchapman/roxie-splitter-selftest

HPCC-12200 Roxie splitters aren't very fair

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>