Depending on the type of test and the product you are developing, your prototyping tool needs different testing characteristics. The minimum is a simulation of scenarios, obviously. The simulation button should allow you to work with the prototype directly in the browser, which allows you to conduct tests remotely or in person. In addition, realistic interaction, device testing capabilities, and data table simulation are also vital if you want users to really try out your end product. If you want to run formal usability tests, you will probably need a prototyping tool that is integrated with a user testing tool.