The money you earn from UserTesting.com is self-employment income. You are in business for yourself, and the income is business income. If you have $400 or more of self-employment income you are required to file a tax return and report the income, as well as any other income that you might have.
Income can come from other things besides working. You can use the calculator at the following link on the IRS web site to see if you have to file a tax return.
Do I Need to File a Tax Return?