There are two strategies you can use to organize your tests. The first is to divide them somehow into separate files, each file representing a functional or logical unit of your application. The other strategy, which can be used in tandem with the first one, is to group them by feature.
Having a separate file for each functional unit of your app is a good way of separating your testing concerns. You should analyze the structure of your application and separate it into distinct concerns that have a minimum amount of overlap. For instance, your application may have to deal with user registrations—that could be one functional group. Another functional group may be user login. If your application deals with to-do lists, you may want to have a separate file that contains the tests for that part of your application.
By having separate files for each functional group, you can call your tests in isolation while you're working on that particular group. This technique also allows...