test/rinda/test_rinda.rb: Prevent a callback Proc from being GC'ed

According to the log of ac803ab55d, I
found that a thread terminates silently due to "recycled object" of
id2ref:

```
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/lib/drb/drb.rb:366:in `_id2ref'"
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/lib/drb/drb.rb:366:in `to_obj'"
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/lib/drb/drb.rb:1528:in `to_obj'"
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/lib/drb/drb.rb:1847:in `to_obj'"
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/lib/drb/drb.rb:1136:in `method_missing'"
"/home/chkbuild/chkbuild/tmp/build/20201017T033002Z/ruby/test/rinda/test_rinda.rb:652:in `block in do_reply'"
```
https://rubyci.org/logs/rubyci.s3.amazonaws.com/rhel8/ruby-master/log/20201017T033002Z.log.html.gz

I believe that this unintentional thread termination has caused
intermittent timeout failure of `TestRingServer#test_do_reply`.

The root cause of the "recycled object" issue is a bug of
`TestRingServer#test_do_reply`.  It creates a callback Proc object but
does not hold the reference to the object:

```
callback = DRb::DRbObject.new callback
```

The original "callback" object is GC'ed unintentionally.
I could consistently reproduce this issue on my machine by adding
`GC.stress = true` at the first of `_test_do_reply` method body.

This change uses another local variable name, "callback_orig", to keep
the original Proc object.

test/rinda/test_rinda.rb

@@ -673,11 +673,11 @@ class TestRingServer < Test::Unit::TestCase
   def _test_do_reply
     called = nil
 
-    callback = proc { |ts|
+    callback_orig = proc { |ts|
       called = ts
     }
 
-    callback = DRb::DRbObject.new callback
+    callback = DRb::DRbObject.new callback_orig
 
     @ts.write [:lookup_ring, callback]