Dagger2 Module class - https://android.jlelse.eu/instrumentation-testing-with-dagger2-retrofit2-mockwebserver-android-4cca7cb7373c

TestApplicationModule.kt

@Module
class TestApplicationModule(application: GitHubDelegate) : ApplicationModule(application) {

    @Singleton
    @Provides
    fun giveRetrofit(okHttpClient: OkHttpClient): Retrofit =
        Retrofit.Builder()
            .baseUrl("http://localhost:8080/")
            .addConverterFactory(GsonConverterFactory.create(GsonBuilder().create()))
            .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
            .client(okHttpClient)
            .build()

    @Singleton
    @Provides
    fun giveMockWebServer(): MockWebServer = MockWebServer()
}