Use require_relative in JSON tests

to prevent them from conflicting with yarp/test_helper

test/json/json_addition_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 require 'json/add/core'
 require 'json/add/complex'
 require 'json/add/rational'

test/json/json_common_interface_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 require 'stringio'
 require 'tempfile'
 

test/json/json_encoding_test.rb

@@ -1,6 +1,6 @@
 # encoding: utf-8
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONEncodingTest < Test::Unit::TestCase
   include JSON

test/json/json_ext_parser_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONExtParserTest < Test::Unit::TestCase
   if defined?(JSON::Ext::Parser)

test/json/json_fixtures_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONFixturesTest < Test::Unit::TestCase
   def setup

test/json/json_generator_test.rb

@@ -2,7 +2,7 @@
 # encoding: utf-8
 # frozen_string_literal: false
 
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONGeneratorTest < Test::Unit::TestCase
   include JSON

test/json/json_generic_object_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONGenericObjectTest < Test::Unit::TestCase
   include JSON

test/json/json_parser_test.rb

@@ -1,6 +1,6 @@
 # encoding: utf-8
 # frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 require 'stringio'
 require 'tempfile'
 require 'ostruct'

test/json/json_string_matching_test.rb

@@ -1,5 +1,5 @@
 #frozen_string_literal: false
-require 'test_helper'
+require_relative 'test_helper'
 require 'time'
 
 class JSONStringMatchingTest < Test::Unit::TestCase

test/json/ractor_test.rb

@@ -1,7 +1,7 @@
 # encoding: utf-8
 # frozen_string_literal: false
 
-require 'test_helper'
+require_relative 'test_helper'
 
 class JSONInRactorTest < Test::Unit::TestCase
   def test_generate