test_weakref.rb: find collected weakref

* test/test_weakref.rb (test_recycled): retry and find a WeakRef
  whose the target is collected, to fix circumstance dependent.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44555 b2dd03c8-39d4-4d8f-98ff-823fe69b080e