Use Mockito to instantly execute handler runnables in your tests.

Instant handler runnable execution with Mockito

// Creating a handler which executes runnables immediately
Mockito.when(uiHandler.post(Mockito.any(Runnable.class))).thenAnswer(
        new Answer<Object>() {
            @Override
            public Object answer(InvocationOnMock invocation) throws Throwable {
                Runnable msg = invocation.getArgument(0);
                msg.run();
                return null;
            }
        }
);