RxJava2 Rule to change the Schedulers to immediate one. (https://www.infoq.com/articles/Testing-RxJava2)

ImmediateSchedulersRule.java

private static class ImmediateSchedulersRule implements TestRule {
   @Override
   public Statement apply(final Statement base, Description description) {
       return new Statement() {
           @Override
           public void evaluate() throws Throwable {
               RxJavaPlugins.setIoSchedulerHandler(scheduler -> 
                                                Schedulers.trampoline());
               RxJavaPlugins.setComputationSchedulerHandler(scheduler -> 
                                                Schedulers.trampoline());
               RxJavaPlugins.setNewThreadSchedulerHandler(scheduler -> 
                                                Schedulers.trampoline());

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