MockWebServer with Retrofit and Dagger

I tried to implement instrumentation test for my android project. In my main project, I used dagger and retrofit and I provide the retrofit from my Module.

This is my Module that provide retrofit:

@Module
public class NetModule {

    @Provides
    @Singleton
    OkHttpClient.Builder provideOkHttpClientBuilder(){
        OkHttpClient.Builder okHttp = new OkHttpClient.Builder();

        HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
        NoCacheInterceptor noCacheInterceptor = new NoCacheInterceptor();
        interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);

        if (!okHttp.interceptors().contains(interceptor))
            okHttp.addInterceptor(interceptor);

        if (!okHttp.interceptors().contains(noCacheInterceptor))
            okHttp.addInterceptor(noCacheInterceptor);

        return okHttp;
    }

    @Provides
    Retrofit provideRetrofit(OkHttpClient.Builder httpClient){
        return new Retrofit.Builder()
              .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
              .addConverterFactory(GsonConverterFactory.create())
              .baseUrl(Constant.BASE_URL)
              .client(httpClient.build()).build();
    }

}

Then, in my androidTest folder, I created TestModule to provide retrofit with MockWebServer like below:

@Module
public class TestNetModule {
    private final MockWebServer mockWebServer;
    public TestNetModule() {
        mockWebServer = new MockWebServer();
    }

    @Provides
    @Singleton
    MockWebServer providerMockWebServer() {
        return new MockWebServer();
    }

    @Provides
    @Singleton
    OkHttpClient.Builder provideOkHttpClientBuilder(){
        OkHttpClient.Builder okHttp = new OkHttpClient.Builder();

        HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
        NoCacheInterceptor noCacheInterceptor = new NoCacheInterceptor();
        interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);

        if (!okHttp.interceptors().contains(interceptor))
            okHttp.addInterceptor(interceptor);

        if (!okHttp.interceptors().contains(noCacheInterceptor))
            okHttp.addInterceptor(noCacheInterceptor);

        return okHttp;
    }

    @Provides
    Retrofit.Builder provideRetrofitBuilder(OkHttpClient.Builder httpClient, MockWebServer mockWebServer){
        return new Retrofit.Builder()
              .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
              .addConverterFactory(GsonConverterFactory.create())
              .baseUrl(mockWebServer.url("/").toString())
              .client(httpClient.build()).build();
    }
}

When I ran my test, I always get error android.os.NetworkOnMainThreadException at code .baseUrl(mockWebServer.url("/").toString()). How is the right implementation MockWebServer with retrofit and dagger for test?