Revert "Dont override global $post variable."

I was wrong in #56. Without declaring the global `$post`, the `setup_postdata()` doesn't work correctly (it needs a reference to the global `$post` variable), meaning template tags like `get_the_ID()` etc. will output the wrong data on the testimonial.

This reverts SHA: 1cd2e6266438851c04c18d68956b7f9f673c3c12 to restore the global declaration.

The issue I had with the global `$post` pointing to the last testimonial found in the subquery is because my request is on an Ajax call, where `wp_reset_postdata()` doesn't have a main query to reset too.