json_pure: fix ractor compatibility

This actually never worked, because the test was always testing
the ext version from the stdlib, never the pure version nor the
current ext version.

Author: byroot

test/json/ractor_test.rb

@@ -9,10 +9,7 @@
 
 class JSONInRactorTest < Test::Unit::TestCase
   def test_generate
-    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
-    begin;
-      $VERBOSE = nil
-      require "json"
+    pid = fork do
       r = Ractor.new do
         json = JSON.generate({
           'a' => 2,
@@ -26,9 +23,22 @@ def test_generate
         })
         JSON.parse(json)
       end
-      expected_json = '{"a":2,"b":3.141,"c":"c","d":[1,"b",3.14],"e":{"foo":"bar"},' +
-                      '"g":"\\"\\u0000\\u001f","h":1000.0,"i":0.001}'
-      assert_equal(JSON.parse(expected_json), r.take)
-    end;
+      expected_json = JSON.parse('{"a":2,"b":3.141,"c":"c","d":[1,"b",3.14],"e":{"foo":"bar"},' +
+                      '"g":"\\"\\u0000\\u001f","h":1000.0,"i":0.001}')
+      actual_json = r.take
+
+      if expected_json == actual_json
+        exit 0
+      else
+        puts "Expected:"
+        puts expected_json
+        puts "Acutual:"
+        puts actual_json
+        puts
+        exit 1
+      end
+    end
+    _, status = Process.waitpid2(pid)
+    assert_predicate status, :success?
   end
-end if defined?(Ractor)
+end if defined?(Ractor) && Process.respond_to?(:fork)