I open example code BasicRxJavaSample (from this article Room+RxJava) The main thing is there:
@Rule
public InstantTaskExecutorRule instantTaskExecutorRule
I think that there is a conflict in some of the linked libraries. I got around this, I used blockingGet() and blockingFirst().
and, in the end, I used https://developer.android.com/training/testing/junit-runner.html#using-android-test-orchestrator
androidTestUtil 'com.android.support.test:orchestrator:1.0.1'
this is what you need!