Inject assert_ractor if TestGemPackageTarHeaderRactor is running under the ruby/rubygems repo

Author: hsbt

test/rubygems/test_gem_package_tar_header_ractor.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
+
 require_relative "package/tar_test_case"
 
+unless Gem::Package::TarTestCase.instance_methods.include?(:assert_ractor)
+  require "core_assertions"
+  Gem::Package::TarTestCase.include Test::Unit::CoreAssertions
+end
+
 class TestGemPackageTarHeaderRactor < Gem::Package::TarTestCase
   ASSERT_HEADERS_EQUAL = <<~RUBY
     def assert_headers_equal(expected, actual)