Mark test_itimer_pthread as flaky. NFC (#21404)

This should have been marked as such when the other itimer tests were.

Author: sbc100

test/test_other.py

@@ -13852,6 +13852,7 @@ def test_itimer(self):
     self.do_other_test('test_itimer.c')
 
   @node_pthreads
+  @flaky('https://github.com/emscripten-core/emscripten/issues/20125')
   def test_itimer_pthread(self):
     self.do_other_test('test_itimer.c')