@st_oehme nailed it one more time 
The following line did the trick.
scopes.TEST.plus += [configurations.integrationRuntimeClasspath, configurations.integrationCompileClasspath]
A little bit outside the original topic but still related, would anyone know why with this integration test configuration, Intellij/AndroidStudio is not able to detect Tests when running it from inside the IDE ?
It executes, but fails with the error Empty test suite.
Thanks again!