[ruby/json] Better handle missing ostruct

In the Ruby test suite, this test class is causing trouble because
ostruct is not available. Having an autoload for JSON::GenericObject but
causing it not to define the constant causes a warning.

See ruby/json@0dc1cd407e77 and
ruby/json@caa5d8cdd748 in ruby.

We can skip defining the test class entirely instead when ostruct is not
available.

ruby/json@6f6a4cdfd7

Author: etiennebarrie

test/json/json_generic_object_test.rb

@@ -1,8 +1,14 @@
 # frozen_string_literal: true
 require_relative 'test_helper'
 
-class JSONGenericObjectTest < Test::Unit::TestCase
+# ostruct is required to test JSON::GenericObject
+begin
+  require "ostruct"
+rescue LoadError
+  return
+end
 
+class JSONGenericObjectTest < Test::Unit::TestCase
   def setup
     if defined?(JSON::GenericObject)
       @go = JSON::GenericObject[ :a => 1, :b => 2 ]