Example wrappers need a top-level initializer

The default initializer made by the compiler takes no arguments.
Use this one to not create a default initializer.

src/spectator/dsl/structure_dsl.cr

@@ -4,6 +4,9 @@ module Spectator
   module DSL
     module StructureDSL
 
+      def initialize(locals : Hash(Symbol, ::Spectator::ValueWrapper))
+      end
+
       macro describe(what, &block)
         context({{what}}) {{block}}
       end