HPCC-9404 Test DEDUP, KEEP(RIGHT)

Added a test (all platforms pass)

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4934 from richardkchapman/dedupright-test

HPCC-9404 Test DEDUP, KEEP(RIGHT)

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>