Prevent Windows-specific unit tests from running on Ubuntu by disabling them.

Add missing file

Author: claudiamurialdo

dotnet/test/DotNetUnitTest/WindowOnlyFact.cs

@@ -0,0 +1,16 @@
+using System.Runtime.InteropServices;
+using Xunit;
+
+namespace DotNetUnitTest
+{
+	public sealed class WindowsOnlyFactAttribute : FactAttribute
+	{
+		public WindowsOnlyFactAttribute()
+		{
+			if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+			{
+				Skip = "Test skipped because it runs only on Windows.";
+			}
+		}
+	}
+}