Use block to create examples

Seems that nodes can't be duped/cloned easily.

src/spectator/example_group.cr

@@ -93,6 +93,13 @@ module Spectator
       group << self if group
     end
 
+    # Creates a child that is attched to the group.
+    # Yields zero or more times to create the child.
+    # The group the child should be attached to is provided as a block argument.
+    def create_child
+      yield self
+    end
+
     # Removes the specified *node* from the group.
     # The node will be unassigned from this group.
     def delete(node : Node)

src/spectator/iterative_example_group.cr

@@ -19,6 +19,13 @@ module Spectator
       end
     end
 
+    # Creates a child that is attched to the group.
+    # Yields zero or more times to create the child.
+    # The group the child should be attached to is provided as a block argument.
+    def create_child
+      @nodes.each { |child| yield child.as(Iteration(T)) }
+    end
+
     # Adds the specified *node* to the group.
     # Assigns the node to this group.
     # If the node already belongs to a group,

src/spectator/spec/builder.cr

@@ -119,12 +119,11 @@ module Spectator
       # It will be yielded two arguments - the example created by this method, and the *context* argument.
       # The return value of the block is ignored.
       # It is expected that the test code runs when the block is called.
-      #
-      # The newly created example is returned.
-      def add_example(name, location, context, metadata = Metadata.new, &block : Example -> _) : Example
+      def add_example(name, location, context, metadata = Metadata.new, &block : Example -> _)
         Log.trace { "Add example: #{name} @ #{location}; metadata: #{metadata}" }
-        Example.new(context, block, name, location, current_group, metadata)
-        # The example is added to the current group by `Example` initializer.
+        current_group.create_child do |group|
+          Example.new(context, block, name, location, group, metadata)
+        end
       end
 
       # Defines a new pending example.
@@ -143,8 +142,9 @@ module Spectator
       # The newly created example is returned.
       def add_pending_example(name, location, metadata = Metadata.new, reason = nil) : Example
         Log.trace { "Add pending example: #{name} @ #{location}; metadata: #{metadata}" }
-        Example.pending(name, location, current_group, metadata, reason)
-        # The example is added to the current group by `Example` initializer.
+        current_group.create_child do |group|
+          Example.pending(name, location, group, metadata, reason)
+        end
       end
 
       # Attaches a hook to be invoked before any and all examples in the current group.