I have temporary solution. The problem seems to be that the android-test-plugin in unable to read an aar file. My solution is to add a dependency to the jar file instead only to testCompile, like this
testCompile files('YOUR_ANDROID_SDK_FOLDER/extras/google/google_play_services/libproject/google-play-services_lib/libs/google-play-services.jar')
Like this I'm able to run a simple test, for example creating a LatLng and checking if it's not null, without errors. Hope it helps.
Edit
A better solution thanks to SuperJugy on github is to apply the fix in this commit https://github.com/SuperJugy/gradle-android-test-plugin/commit/d5b7836cf7278dc9d0fe765b635de60c33911d72