[ruby/prism] Fix fork check in ractor_test.rb

ruby/prism@0073266cad

Author: eregon

test/prism/ractor_test.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-return if !defined?(Ractor) || !defined?(fork)
+return unless defined?(Ractor) && Process.respond_to?(:fork)
 
 require_relative "test_helper"