test_weakref.rb: use single level WeakRef

* test/test_weakref.rb (make_weakref): return single level
  WeakRef, instead of multiple level, since the second level
  object may be still alive even after the final object get
  finalized, in test_recycled.

* test/test_weakref.rb (test_ref): move living object assertions
  from test_recycled.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44146 b2dd03c8-39d4-4d8f-98ff-823fe69b080e