Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

src/Spectre.Console.Testing/Cli/CallbackCommandInterceptor.cs

@@ -1,6 +1,3 @@
-using System;
-using Spectre.Console.Cli;
-
 namespace Spectre.Console.Testing;
 
 /// <summary>
@@ -24,4 +21,4 @@ public sealed class CallbackCommandInterceptor : ICommandInterceptor
     {
         _callback(context, settings);
     }
-}
+}