Example group builder classes should build groups

It's implied in their name.

src/spectator/dsl/builder.cr

@@ -5,6 +5,10 @@ module Spectator
 
       @@group_stack = [::Spectator::DSL::ExampleGroupBuilder.new("ROOT")]
 
+      private def root_group
+        @@group_stack.first
+      end
+
       private def current_group
         @@group_stack.last
       end
@@ -53,9 +57,8 @@ module Spectator
         current_group.add_around_each_hook(block)
       end
 
-      protected def build : Array(Example)
-        # TODO
-        [] of Example
+      protected def build : ExampleGroup
+        root_group.build({} of Symbol => ValueWrapper)
       end
     end
   end

src/spectator/dsl/example_group_builder.cr

@@ -35,9 +35,8 @@ module Spectator
         @around_each_hooks << block
       end
 
-      def build : Array(Example)
-        # TODO
-        Array(Example).new
+      def build(locals : Hash(Symbol, ValueWrapper)) : ExampleGroup
+        raise NotImplementedError
       end
     end
   end

src/spectator/dsl/given_example_group_builder.cr

@@ -8,9 +8,8 @@ module Spectator
         super(what)
       end
 
-      def build : Array(Example)
-        # TODO
-        Array(Example).new
+      def build(locals : Hash(Symbol, ValueWrapper)) : ExampleGroup
+        raise NotImplementedError
       end
     end
   end