Only test Ractor sharability if Ractors are defined

Author: tenderlove

test/test_statement.rb

@@ -25,8 +25,10 @@ def test_rows_should_be_frozen
       assert_predicate row, :frozen?
       row.each { |item| assert_predicate item, :frozen? }
 
-      assert Ractor.shareable?(rows)
-      assert Ractor.shareable?(row)
+      if defined?(Ractor)
+        assert Ractor.shareable?(rows)
+        assert Ractor.shareable?(row)
+      end
     end
 
     def test_double_close_does_not_segv