Yes, you’ll need a valid license for every Envato item you use in making your test site. For themes and plugins the Regular License will be fine if the site is never made accessible to the public. Photos, fonts, or anything else you use in the test site will also need to be properly licensed.
If you find yourself making a lot of test sites, consider Envato Elements. For a monthly fee you can download and use as many items as you want - perfect for test sites!