Add missing GivenExampleGroupBuilder class

src/spectator/dsl/given_example_group_builder.cr

@@ -0,0 +1,17 @@
+require "./example_group_builder"
+
+module Spectator
+  module DSL
+    class GivenExampleGroupBuilder < ExampleGroupBuilder
+
+      def initialize(what : String, @collection : Array(ValueWrapper))
+        super(what)
+      end
+
+      def build : Array(Example)
+        # TODO
+        Array(Example).new
+      end
+    end
+  end
+end