rcutorture: Don't create results directory for dryruns

This commit prevents the results directory from being created for
dryruns.  However, a script generated from a dryrun will create
the results directory should it be run.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>