public void ShouldDownloadPage()
{
//Arrange
string url = "https://en.m.wikipedia.org/wiki/Medicine";
string filename = "file1.html";
DownloadService downloadService = Substitute.For <DownloadService>();
//Act
downloadService.DownloadHtml(url, filename);
//Assert
downloadService.Received(1).DownloadHtml(url, filename);
}