AccessTokenCreator.java

package com.facebook;

import java.util.ArrayList;
import java.util.Collection;

public class AccessTokenCreator {

    public static AccessToken createToken(Collection<String> grantedPermissions) {
        return new AccessToken("token", "appId", "userId", grantedPermissions,
                new ArrayList<String>(), AccessTokenSource.WEB_VIEW, null, null);
    }
}

LoginClientCreator.java

package com.facebook.login;

import android.support.test.espresso.core.deps.guava.collect.Sets;

import com.facebook.AccessTokenCreator;
import com.facebook.login.LoginClient.Request;
import com.facebook.login.LoginClient.Result;

import java.util.HashSet;

public class LoginClientCreator {

    public static Request createRequest() {
        final HashSet<String> permissions = Sets.newHashSet("user_actions.music", "user_friends", "user_likes", "email");
        return new Request(LoginBehavior.NATIVE_WITH_FALLBACK, permissions, DefaultAudience.EVERYONE, "appId", "authId");
    }

    public static Result createResult() {
        final Request request = createRequest();
        return new Result(request, Result.Code.SUCCESS,  AccessTokenCreator.createToken(request.getPermissions()), null, null);
    }
}

LoginSignupActivityTest.java

@Test
public void signupWithFacebook() {
    Context context = InstrumentationRegistry.getInstrumentation().getContext();

    final Intent resultData = new Intent();
    resultData.putExtra("com.facebook.LoginFragment:Result", LoginClientCreator.createResult());
    intending(hasComponent(FacebookActivity.class.getName()))
            .respondWith(new Instrumentation.ActivityResult(Activity.RESULT_OK, resultData));

    activityTestRule.launchActivity(LoginSignupActivity.buildIntent(context));

    // click signup fb button
    onView(withId(R.id.signup_with_facebook_button))
            .perform(click());

    // check facebook intent was hit
    intended(hasComponent(FacebookActivity.class.getName()), times(1));

    // check UI is showing the user as logged in
    ...
}

I have added a Kotlin version with some updates here: https://gist.github.com/kenilt/df29ff41943ed1ca0ba8ae4f85e5ed0a