Android test loading file from res/raw

I want to test that a file is loaded from res/raw but can't seem to get it working.

The class I'm testing is:

public class SomeService extends ContextWrapper {

    public SomeService(Context base) {
        super(base);
    }

    public BufferedReader loadSuperFile() {
        InputStream inputStream = getResources().openRawResource(R.raw.superFile);
        return new BufferedReader(new InputStreamReader(inputStream));
    }
}

With the test class being:

@Test
public void aTest() {
    SomeService someService = new SomeService(new MainActivity().getApplicationContext());
    BufferedReader br = someService.loadSuperFile();
}

But I get a RuntimeException: Method getApplicationContext in android.content.ContextWrapper not mocked.

I suspect it's the context but just can't find the magic formula. Any help would be much appreciated :)