In upgrading to 2.15.0, bxSlider will not be enabled by default. You must enable it in your widget and global settings.
Further, any of your Testimonials Widget CSS customizations must be reviewed to have the `.active` and `.display-none` classes removed. The main `.testimonials-widget-testimonial` class also needs the `display: none;` and `clear: left;` removed.
If this still doesn't work, refer to testimonials-widget.css for comparison.