test/ruby/test_thread_queue.rb: add a wait to yield a thread

On Arch, sending a signal in a loop seems hardly to yield the execution.

```
  1) Error:
TestThreadQueue#test_queue_with_trap:
Timeout::Error: execution of assert_in_out_err expired timeout (10 sec)
```
20200209T120002Z.fail.html.gz

This change allows the test to pass.

test/ruby/test_thread_queue.rb

@@ -572,6 +572,7 @@ class TestThreadQueue < Test::Unit::TestCase
       Thread.new{
         loop{
           Process.kill :INT, $$
+          sleep 0.1
         }
       }
       puts q.pop