Shortcodes: Testimonial: add "id" attribute

Add the ability to return specific posts when using a shortcode to display a [testimonials] Custom Content Type.

Current attributes are only: display_content, image, columns, showposts, order, orderby, date
https://github.com/Automattic/jetpack/blob/master/modules/custom-post-types/testimonial.php#L528

Can we add an ID attribute?

Requested by a user in 1246884-zen

Start in modules/custom-post-types/testimonial.php around line 528, where the testimonial shortcode attributes are defined. Trace how the existing attributes select posts and add an id attribute that returns the requested testimonial posts. Verify the shortcode still supports the current attributes and that an ID selects the intended post.

Written by the indexing model from the issue text.