title: Mock ApolloClient's query(call) for testing purposes
body: <p>Kotlin Android app using graphQl &amp; ApolloClient.</p>
  <p>I want to test this method:</p>
  <pre><code>fun myMethod(id: String): Single&lt;List&lt;MyEntity&gt;&gt; {
      val call = Query(id)
      return Rx2Apollo.from(apolloClient.query(call))
        .map { modeCode() }.firstOrError().onErrorReturn { emptyList() }
    }
  </code></pre>
  <p>Is there an elegant way to mock the response of <code>apolloClient.query(call)</code> so Rx2Apollo returns the response?</p>
  <p>I tried with a RealApolloCall, with a custom ApolloCall, etc, nothing convinces me.
  And I wouldn't want to extract and mock the whole <code>Rx2Apollo.from(apolloClient.query(call))</code> part. To use mockWebServer doesn't apply, because I do not want to test apolloClient, I want to mock its response to test the rest.</p>
  <p>There is a related issue that was closed without really mentioning the best way to solve:
  <a href="https://github.com/apollographql/apollo-android/issues/1144" rel="noreferrer">https://github.com/apollographql/apollo-android/issues/1144</a></p>
  <p>Thanks!</p>