stop Ractor test in test-all

Ractor changes the interpreter's running mode so now it should
not use in test-all process which running with many other tests.

Test with a separating process is one idea, but I'm not sure
the ruby/ostruct can use this trick.

test/ostruct/test_ostruct.rb

@@ -300,6 +300,8 @@ def []=(k, v)
     assert_equal 42, o.foo
   end
 
+=begin
+  # now Ractor should not use in test-all process
   def test_ractor
     obj1 = OpenStruct.new(a: 42, b: 42)
     obj1.c = 42
@@ -310,6 +312,7 @@ def test_ractor
     end.take
     assert obj1.object_id == obj2.object_id
   end if defined?(Ractor)
+=end
 
   def test_legacy_yaml
     s = "--- !ruby/object:OpenStruct\ntable:\n  :foo: 42\n"