Adding System.Interactive.Async.Tests folder structure.

Ix.NET/Source/System.Interactive.Async.Tests/System.Interactive.Async.Tests.csproj

@@ -53,4 +53,8 @@
     </Compile>
   </ItemGroup>
 
+  <ItemGroup>
+    <Folder Include="System\Linq\Operators\" />
+  </ItemGroup>
+
 </Project>