You can put them in the same module as the sources of your apk. Create a directory structure as follows:
-<Project root> |-<Module name> |-src |-main |-com |-<company name> |-<app name> |-instrumentTest |-com |-<company name> |-<app name> <- Put your unittests here
You can launch them by creating a new launch configuration in Android Studio. Select a new Android test run.
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