Ryan Ron Stack Overflow • Recommends
On “Android Studio / Gradle test folder naming convention”
Since Android Studio 1.1 you should put your Unit tests in /src/test and Android Instrumentation Tests in /src/androidTest
See: http://tools.android.com/tech-docs/unit-testing-support#TOC-Setting-up-Android-Studio
For Android Studio 2.0 and beyond see: link to this video