[ruby/open3] Only use JITSupport on CRuby

Fixes #2

https://github.com/ruby/open3/commit/6b7ede69e8

test/test_open3.rb

@@ -2,7 +2,10 @@
 
 require 'test/unit'
 require 'open3'
-require_relative 'lib/jit_support'
+
+if RUBY_ENGINE == 'ruby'
+  require_relative 'lib/jit_support'
+end
 
 class TestOpen3 < Test::Unit::TestCase
   RUBY = EnvUtil.rubybin
@@ -127,7 +130,11 @@ class TestOpen3 < Test::Unit::TestCase
           i.close
           STDERR.reopen(old)
           assert_equal("zo", o.read)
-          assert_equal("ze", JITSupport.remove_mjit_logs(r.read))
+          if defined?(JITSupport)
+            assert_equal("ze", JITSupport.remove_mjit_logs(r.read))
+          else
+            assert_equal("ze", r.read)
+          end
         }
       }
     }