What you are trying to do is not going to work the way you are trying to do it because on post save WordPress will process the data and then redirect back to the originating page. That means that any global you set before the redirect isn’t going to be there afterwards.
However, your custom meta information should be retrieved with this– $values = get_post_custom( $post->ID );— so your should have no trouble having that information to use in your testimonial_text_box_content and the same line of code would get the values (all custom post meta) inside the Loop on the front end.
When you echo your textarea you want to include your previous content between the opening and closing tags, like with paragraph tag. It is different in this way from an input form element.
echo '<textarea align="top" id="testimonial_text" name="testimonial_text" value="'.esc_attr($value).'" style="width:100%;height:200px;margin:5px -20px 3px 0;" />'.$values['testimonial_text'].'</textarea>';
I am working from memory here and can’t test at the moment, so there may be bugs in that but hopefully it will move you forward.
Other than that, I didn’t test the code but I don’t spot anything obviously wrong, by the way.