HPCC-9070 False positive from dict15c test on thor

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4191 from richardkchapman/regression-dict15

HPCC-9070 False positive from dict15c test on thor

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>