For testing, you have to "serialize threads" of RX. Regardless of Scheduler production code specifies, you have to execute that instantly. That is why we are using custom rule. In it we override any new schedulers with immediate one.

RxImmediateSchedulerRule.java

package com.eutechpro;


import org.junit.rules.TestRule;
import org.junit.runner.Description;
import org.junit.runners.model.Statement;

import java.util.concurrent.TimeUnit;

import io.reactivex.Scheduler;
import io.reactivex.android.plugins.RxAndroidPlugins;
import io.reactivex.annotations.NonNull;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.schedulers.ExecutorScheduler;
import io.reactivex.plugins.RxJavaPlugins;

public class RxImmediateSchedulerRule implements TestRule {
    private Scheduler immediate = new Scheduler() {
        @Override
        public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
            return super.scheduleDirect(run, 0, unit);
        }

        @Override
        public Scheduler.Worker createWorker() {
            return new ExecutorScheduler.ExecutorWorker(Runnable::run);
        }
    };

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                RxJavaPlugins.setInitIoSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitComputationSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitNewThreadSchedulerHandler(scheduler -> immediate);
                RxJavaPlugins.setInitSingleSchedulerHandler(scheduler -> immediate);
                RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> immediate);

                try {
                    base.evaluate();
                } finally {
                    RxJavaPlugins.reset();
                    RxAndroidPlugins.reset();
                }
            }
        };
    }
}