It's best not to edit the core of a plugin if you can avoid it. Your changes will be overwritten when you update. There is another method you can use that seems to have more options than the widget provides.
Add the shortcode to a text widget.
[testimonial_rotator id=121 hide_title="true"]
You may need to add this to your functions.php to enable shortcodes in widgets.
Using this method it also picked up the scrolling transition effect I had in the settings. Using the widget it only seems to use fade. I would like to see a list off all the shortcode attributes.
Another option would be to ask the author to add the option to the rotator settings and/or the widget settings. :) Please. :)