Mark test_itimer_proxy_to_pthread as flaky. NFC (#21175)

This test recently flaked on window too.

Author: sbc100

test/test_other.py

@@ -13722,7 +13722,7 @@ def test_itimer_standalone(self):
       self.assertContained('done', self.run_js('test_itimer_standalone.wasm', engine))
 
   @node_pthreads
-  @no_mac("Our Mac CI currently has too much contention to run this reliably")
+  @flaky('https://github.com/emscripten-core/emscripten/issues/20125')
   def test_itimer_proxy_to_pthread(self):
     self.set_setting('PROXY_TO_PTHREAD')
     self.set_setting('EXIT_RUNTIME')