Fix harness-common.rb to not depend on Ractor being defined

* Ractor is only defined on CRuby.

Author: eregon

harness/harness-common.rb

@@ -13,7 +13,7 @@
 # Seed the global random number generator for repeatability between runs
 Random.srand(1337)
 
-if !Ractor.respond_to?(:make_shareable)
+if defined?(Ractor) && !Ractor.respond_to?(:make_shareable)
   class Ractor
     #noop
     def make_shareable(obj, copy: false); obj; end