Sort and remove using directives in tests as well, I missed these

Author: chuggafan

RedditSharpTests/Things/PostTests.cs

@@ -1,5 +1,4 @@
 using RedditSharp.Things;
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;

RedditSharpTests/Things/SubredditTests.cs

@@ -1,6 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+using System.Linq;
 using System.Threading.Tasks;
 using Xunit;
 

RedditSharpTests/Wiki/WikiTests.cs

@@ -1,6 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+using System.Linq;
 using System.Threading.Tasks;
 using Xunit;
 namespace RedditSharpTests.Wiki