Android Studio Tests are successful but do not stop junit

I am new to Android and building an app in Android Studio. I have written some tests using JUnit and Mockito. The tests run successfully and in the run console the text "Process finished with exit code 0" is shown. However, the thread seems to remain active, the stop button is enabled and nothing happens even if I click on it. If I try to start multiple test configurations, they all remain active although the processes finish successfully. I hope the question is clear enough... I have searched a lot for the answer and mananged to find nothing.