Don't move the timer_thread when it's enclosed

Don't move the timer_thread to ThreadGroup::Default, when it's
created in an enclosed ThreadGroup.
Prevents the exception: "add" can't move from the enclosed thread group"

lib/timeout.rb

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

test/test_timeout.rb

@@ -172,4 +172,25 @@ def test_threadgroup
     end;
   end
 
+  def test_handling_enclosed_threadgroup
+    # The problem "add: can't move from the enclosed thread group" #24,
+    # happens when the timeout_thread is created in an enclosed ThreadGroup.
+    assert_separately(%w[-rtimeout], <<-'end;')
+      Timeout.instance_eval { @timeout_thread = nil }
+
+      t1 = Thread.new {
+        Thread.stop
+        assert_block do
+          Timeout.timeout(0.1) {}
+          true
+        end
+      }
+      sleep 0.1 while t1.status != 'sleep'
+      group = ThreadGroup.new
+      group.add(t1)
+      group.enclose
+      t1.run
+      t1.join
+    end;
+  end
 end