test_objectspace.rb: run in separate process

* test/ruby/test_objectspace.rb (TestObjectSpace#test_finalizer): run
  in separate process so that finalizers run at termination certainly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43626 b2dd03c8-39d4-4d8f-98ff-823fe69b080e