Passing an editing test is largely a matter of proficiency. Depending on the level of editing required and the type of material you'll be working with, you must be knowledgeable in many aspects of the craft.
Start by familiarizing yourself with the basics of grammar, punctuation, and spelling, as well as the style guide associated with the material. This guide will provide the governing rules for editing the project at hand. Take some time to get comfortable with the syntax and structure of the guide, and double-check the formatting of your work.
Next, be sure to take the time to read each and every piece of content all the way through, paying particularly close attention to the most minute of details. The same goes for any images or other visual components: make sure that each one is of the highest quality, with no errors or defects.
If you find yourself stuck or tripped up by a certain aspect of the material, make sure to take notes and ask questions. Don't hesitate to ask for help from colleagues or advisors if the task is outside of your wheelhouse: doing so will ensure that your work is as high-quality as possible.
By keeping these tips in mind, you ensure that the project you're editing is done so concisely and professionally, while also passing the editing test!