DTest's exit code should be configureable

We use 20/21 to indicate success/failure (based on experience showing that a 0 could also be a sign of "app did not set explicit return code" or "silent death" etc.)
OTOH, @xpqz argues "_GitHub Actions expect the "standard" unix convention that 0 is success, non-zero is an error code.
I think it would be nice if dtest could be told to return 0 for success. A lot of tools expect that._"

NB: current behavior should not be changed - but additional options should be introduced to enable alternative values.