Fix busted compile on Linux

Author: ericsciple

src/Test/L0/Listener/Configuration/ConfigurationManagerL0.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Runtime.CompilerServices;
 using System.Security.Cryptography;
 using System.Threading.Tasks;