In a project by default on Android Studio there are two test folders.
The first is
testCompile 'junit: junit: 4.12'
src/androidTest is for unit tests that involves android instrumentation.
src/test is for pure unit test that do not involve android framework. You can run tests here without running on a real device or on emulator.
You can use both folders. Use the first one to test code that use Android framework. Use the second one to test code that are pure java classes. The methods to write tests are almost the same.
More info here : http://developer.android.com/tools/testing/testing_android.html