Explicit add the timeout thread to default ThreadGroup

Otherwise the timeout thread would be added to the ThreadGroup of the thread that makes the first call to Timeout.timeout .

Fixes bug 19020: https://bugs.ruby-lang.org/issues/19020

Add a test case to make sure the common thread doesn't leak to another ThreadGroup

Author: larskanis

lib/timeout.rb

@@ -120,6 +120,7 @@ def self.create_timeout_thread
         requests.reject!(&:done?)
       end
     end
+    ThreadGroup::Default.add(watcher)
     watcher.name = "Timeout stdlib thread"
     watcher.thread_variable_set(:"\0__detached_thread__", true)
     watcher

test/test_timeout.rb

@@ -159,4 +159,17 @@ def test_fork
     assert_equal 'timeout', r.read
     r.close
   end
+
+  def test_threadgroup
+    assert_separately(%w[-rtimeout], <<-'end;')
+      tg = ThreadGroup.new
+      thr = Thread.new do
+        tg.add(Thread.current)
+        Timeout.timeout(10){}
+      end
+      thr.join
+      assert_equal [].to_s, tg.list.to_s
+    end;
+  end
+
 end