Update XamlIslandsTest_StringExtensions to fix from Extensions namespace change

Related to #3743

Author: michael-hawker

UnitTests/UnitTests.XamlIslands.UWPApp/XamlIslandsTest_StringExtensions.cs

@@ -23,7 +23,7 @@ public async Task StringExtensions_GetViewLocalized()
             await App.Dispatcher.EnqueueAsync(() =>
             {
                 var xamlRoot = App.XamlRoot;
-                var str = StringExtensions.GetViewLocalized("abc", xamlRoot.UIContext);
+                var str = "abc".GetViewLocalized(xamlRoot.UIContext);
                 Assert.AreEqual("ABCDEF", str);
             });
         }
@@ -136,4 +136,4 @@ await App.Dispatcher.EnqueueAsync(async () =>
             });
         }
     }
-}
+}