How to wait for async task in Espresso without IdlingResource

I have the following espresso code:

@Test
fun backgroundWorkDisplaysTextAfterLoading() {
    onView(withId(R.id.btn_next_fragment)).perform(click())
    onView(withId(R.id.btn_background_work)).perform(click())

    onView(withId(R.id.hiddenTextView)).check(matches(isDisplayed()))
}

When the btn_background_work is clicked, there is a service call that will return a response in 2 seconds and then the hiddenTextview will become visible.

How can I let Espresso wait to execute the ViewAssertion ( check(matches(isDisplayed)) ) until this service call has returned a value? The service call is done with Retrofit and the service call is done on the Schedulers.io() thread.

I cannot touch the production code, so implementing IdlingResources in the production code is impossible.

Any help is appreciated!