Generally, IntelliJ IDEA runs and debugs tests in the same way as other applications, by running the run/debug configurations you have created. When doing so, it passes the specified test classes or methods to the test runner.
If you run a test for which there is no permanent run/debug configuration, a temporary configuration is created. You can then save such a configuration using the Run/debug configuration dialog if you want to reuse it later.
The tests run in the background, so you can execute several tests at the same time.
Run or debug a test
To start running or debugging a test, you can use the main toolbar or a context menu in the Project tool window or in the editor:
Use the main toolbar:
Select the necessary run/debug configuration from the list on the main toolbar.
Press Shift+Alt+F10 to see the list of available run configurations or Shift+Alt+F9 for debug configurations.
Click Run or Debug to the right of the list. Alternatively, select Shift+F10 or Shift+F9 from the main menu.
Use a context menu:
Right-click a test class in the Project tool window or open it in the editor, and right-click the background. From the context menu, select Run <class name> or Debug....
For a test method, open the class in the editor and right click anywhere in the method. The context menu suggests the command Run / Debug <method name>.