Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

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

@@ -1,6 +1,3 @@
-using System;
-using Spectre.Console.Cli;
-
 namespace Spectre.Console.Testing;
 
 /// <summary>
@@ -131,4 +128,4 @@ public sealed class CommandAppTester
 
         return new CommandAppResult(result, output, context, settings);
     }
-}
+}