test/rinda/test_rinda.rb: try debugging TestRingServer#test_do_reply

https://rubyci.org/logs/rubyci.s3.amazonaws.com/rhel8/ruby-master/log/20201016T063003Z.fail.html.gz
```
  1) Error:
Rinda::TestRingServer#test_do_reply:
Timeout::Error: timeout
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:837:in `sleep'
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:837:in `wait_for'
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:659:in `_test_do_reply'
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:643:in `block in test_do_reply'
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:807:in `with_timeout'
    /home/chkbuild/chkbuild/tmp/build/20201016T063003Z/ruby/test/rinda/test_rinda.rb:643:in `test_do_reply'
```

1 files changed, 8 insertions, 1 deletions

diff --git a/test/rinda/test_rinda.rb b/test/rinda/test_rinda.rb
index 14fc7ef2cc..1991ab6abf 100644
--- a/test/rinda/test_rinda.rb
+++ b/test/rinda/test_rinda.rb
@@ -620,6 +620,7 @@ end
 class TestRingServer < Test::Unit::TestCase
 
   def setup
+    @aoe_back = Thread.abort_on_exception
     @port = Rinda::Ring_PORT
 
     @ts = Rinda::TupleSpace.new
@@ -627,6 +628,7 @@ class TestRingServer < Test::Unit::TestCase
     @server = DRb.start_service("druby://localhost:0")
   end
   def teardown
+    Thread.abort_on_exception = @aoe_back
     @rs.shutdown
     # implementation-dependent
     @ts.instance_eval{
@@ -800,7 +802,12 @@ class TestRingServer < Test::Unit::TestCase
     tl = nil
     th = Thread.new(Thread.current) do |mth|
       sleep n
-      (tl = Thread.list - tl0).each {|t|t.raise(Timeout::Error)}
+      puts "...timeout! Show the backtraces of all living threads"
+      (tl = Thread.list - tl0).each_with_index do |t, i|
+        puts "Thread #{ i }: #{ t.inspect }", *t.backtrace, ""
+        t.raise(Timeout::Error)
+      end
+      puts "and then raise Timeout::Error to the main thread"
       mth.raise(Timeout::Error)
     end
     tl0 << th