Simplify test

test/test_timeout.rb

@@ -172,23 +172,17 @@ def test_threadgroup
     end;
   end
 
+  # https://github.com/ruby/timeout/issues/24
   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;')
-      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
+      Thread.new {
+        t = Thread.current
+        group = ThreadGroup.new
+        group.add(t)
+        group.enclose
+
+        assert_equal 42, Timeout.timeout(1) { 42 }
+      }.join
     end;
   end
 end