HPCC-10877 More issues in stepped keyed limits

The steplimit regression tests fail on single-part indexes when
allFilesDynamic=false

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5422 from richardkchapman/steplimit-assert

HPCC-10877 More issues in stepped keyed limits

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>