PowerMock + Robolectric

MainActivity.java

package com.hkurokawa.powermocktest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

import rx.android.schedulers.AndroidSchedulers;
import rx.functions.Action1;
import rx.schedulers.Schedulers;

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MyStaticClass.getObservable()
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Action1<Void>() {
                    @Override
                    public void call(Void aVoid) {
                        startActivity(AnotherActivity.newInstance(MainActivity.this));
                    }
                }, new Action1<Throwable>() {
                    @Override
                    public void call(Throwable throwable) {
                        Log.e(TAG, "Failed: " + throwable.getMessage(), throwable);
                    }
                });
    }
}

MainActivityTest.java

package com.hkurokawa.powermocktest;

import android.content.Intent;

import org.junit.Rule;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PowerMockIgnore;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.rule.PowerMockRule;
import org.robolectric.Robolectric;
import org.robolectric.RobolectricGradleTestRunner;
import org.robolectric.Shadows;
import org.robolectric.annotation.Config;
import org.robolectric.shadows.ShadowActivity;

import rx.subjects.PublishSubject;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;

@RunWith(RobolectricGradleTestRunner.class)
@Config(constants = BuildConfig.class, sdk = 21)
@PowerMockIgnore({"org.mockito.*", "org.robolectric.*", "android.*"})
@PrepareForTest({MyStaticClass.class})
public class MainActivityTest {
    // Robolectric, Using Power Mock https://github.com/robolectric/robolectric/wiki/Using-PowerMock
    @Rule
    public PowerMockRule rule = new PowerMockRule();

    @Test
    public void test() {
        PowerMockito.mockStatic(MyStaticClass.class);

        final PublishSubject<Void> dummyObservable = PublishSubject.create();
        PowerMockito.when(MyStaticClass.getObservable()).thenReturn(dummyObservable.asObservable());

        final MainActivity activity = Robolectric.setupActivity(MainActivity.class);
        final ShadowActivity shadowOfActivity = Shadows.shadowOf(activity);

        dummyObservable.onNext(null);
        final Intent intent = shadowOfActivity.getNextStartedActivity();
        assertNotNull(intent);
        assertEquals(intent.getComponent().getClassName(), AnotherActivity.class.getName());
    }
}

MyStaticClass.java

package com.hkurokawa.powermocktest;

import rx.Observable;

class MyStaticClass {
    static Observable<Void> getObservable() {
        return null;
    }
}