public void Test_KeyExists_Method()
{
Favourites h = Favourites.InstanceNoFileWrite;
h.AddEntry("http://www.duckduckgo.com", "DuckDuckGo", false);
Assert.ThrowsException <System.ArgumentException>(() => h.AddEntry("http://www.duckduckgo.com", "DuckDuckGo", false));
}