Add debug message for test_rinda crash

We often see test-all worker crash on test_rinda, but for now we even
can't know which test is unstable from this output:
http://ci.rvm.jp/results/trunk_gcc5@silicon-docker/1926481

Let me print `caller` on the timeout failure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67463 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rinda/test_rinda.rb

@@ -801,6 +801,8 @@ class TestRingServer < Test::Unit::TestCase
     tl0 << th
     yield
   rescue Timeout::Error => e
+    $stderr.puts "TestRingServer#with_timeout: timeout in #{n}s:"
+    $stderr.puts caller
     if tl
       bt = e.backtrace
       tl.each do |t|