Added snapshots and excluded Windows form tests

Author: kubaflo

src/Controls/tests/TestCases.Android.Tests/snapshots/android/CustomBackButtonShouldBeRed.png

@@ -1,4 +1,5 @@
-using NUnit.Framework;
+#if ANDROID || IOS
+using NUnit.Framework;
 using UITest.Appium;
 using UITest.Core;
 
@@ -19,4 +20,5 @@ public void CustomBackButtonShouldBeRed()
 		App.WaitForElement("label");
 		VerifyScreenshot();
 	}
-}
+}
+#endif