To test app components in a non-default processes, you can use the functionality of Multiprocess Espresso. This tool, available on Android O (API level 26) and higher, allows you to seamlessly test your app's UI interactions that cross your app's process boundaries while maintaining Espresso's synchronization guarantees.
The application will be started on the device/emulator and a series of actions will be performed automatically.
If you are using Android Studio, the Run window will show the test results.
Activity running in the default process which matches the application package
Activity running in a private process which is manually defined in the AndroidManifest.xml using the android:process attribute.
From D.A.C: "If the name assigned to this attribute begins with a colon (':'), a new process, private to the application, is created when it's needed and the activity runs in that process."