(GH-1020) WindowsOnly tests by platform, not ignore

Previously, Windows only tests were ignored based on a build compilation
symbol `__MonoCS__`. This allowed skipping (ignoring) those tests
easily. However newer versions of Mono simply don't use that symbol
anymore and the testing aspect does not ignore those tests, ultimately
causing them to fail when run in a non-Windows environment.

Switch over to using platform that only includes Windows. This will
exclude Windows only tests from running on Unix, Linux and MacOS.

src/chocolatey.tests/TinySpec.cs

@@ -147,18 +147,13 @@ public PendingAttribute(string reason)
         }
     }
 
-#if __MonoCS__
-     public class WindowsOnlyAttribute : IgnoreAttribute
+    public class WindowsOnlyAttribute : PlatformAttribute
     {
-        public WindowsOnlyAttribute() : base("This is a Windows only test")
+        public WindowsOnlyAttribute()
         {
+            Exclude = "Unix,Linux,MacOsX";
         }
     }
-#else
-    public class WindowsOnlyAttribute : Attribute
-    {
-    }
-#endif
 
     public class IntegrationAttribute : CategoryAttribute
     {