How can I copy media file to camera roll used ALAssetsLibrary in unit test

I want to copy a media file from Resource folder to camera roll in unit test.

Here my code :

-(NSString *)copyFileToCameraRool:(NSString *)fileName{
   __block NSString* ret;
    if (fileName != nil && [fileName length] > 0 ) {
        UIImage *image = [UIImage imageNamed:fileName];
        UIImageWriteToSavedPhotosAlbum(image, nil,nil, nil);
        ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
        // Request add image to camera roll
         XCTestExpectation *mXCTestExpectation = [self expectationWithDescription:@"XCTestExpectation"];
        [library writeImageToSavedPhotosAlbum:[image CGImage] orientation:(ALAssetOrientation)[image imageOrientation] completionBlock:^(NSURL *assetURL, NSError *error){
            if (error) {
                NSLog(@"error");
            } else {
                NSLog(@"url %@", assetURL);
            }
            [mXCTestExpectation fulfill];
            ret = [assetURL absoluteString];
        }];
    }
    [self waitForExpectationsWithTimeout:10 handler:NULL];
    return ret;
}

When I'm running this code, an error occurred "[0] (null) @"NSLocalizedFailureReason" : @"The user has denied the application access to their media.".

Can anyone help me solve this problem ?

Thanks you.