Add global usings (#668)

* Use global usings

* Fix namespace declarations for test projects

src/Spectre.Console.Testing/Extensions/StringExtensions.cs

@@ -1,5 +1,3 @@
-using System.Collections.Generic;
-
 namespace Spectre.Console.Testing;
 
 /// <summary>
@@ -43,4 +41,4 @@ public static class StringExtensions
 
         return string.Empty;
     }
-}
+}

src/Spectre.Console.Testing/Extensions/StyleExtensions.cs

@@ -21,4 +21,4 @@ public static class StyleExtensions
 
         return style.Background(color);
     }
-}
+}