Make Example#group abstract

Instead of passing parent group as an argument, define the group method 
in the exapmle class created by the #it macro.

src/spectator/dsl/structure_dsl.cr

@@ -140,10 +140,13 @@ module Spectator
           def description
             {{description.is_a?(StringLiteral) ? description : description.stringify}}
           end
+
+          def group
+            ::Spectator::Definitions::GROUPS[{{@type.symbolize}}]
+          end
         end
 
-        %group = ::Spectator::Definitions::GROUPS[\{{@type.symbolize}}]
-        %group.children << Example%example.new(%group)
+        ::Spectator::Definitions::GROUPS[\{{@type.symbolize}}].children << Example%example.new
       end
 
       macro pending(description, &block)

src/spectator/example.cr

@@ -1,12 +1,9 @@
 module Spectator
   abstract class Example
-    getter group : ExampleGroup
     getter? finished = false
 
-    def initialize(@group)
-    end
-
     abstract def run : Result
     abstract def description : String
+    abstract def group : ExampleGroup
   end
 end