Fix flakiness in test_itimer_proxy_to_pthread

[sbc100]

Followup to #27778.

Calling printf in these signal handlers was causing issues since it requires taking the stdio lock which in turn can run more proxied work (signal handler all run on the main thread). In this case, the flake is caused by the setitimer which cancels the timers running during the printf call (and before the counter is incremented), i.e. in the middle of the signal handler. This means that worker thread can observe the effect of the timer (the counter being incremented) after the timer is cancelled (because the cancellation effectively ran during the execution of the signal handler).

Possible followups to avoid this type of thing in the future:

1. Mask signals during handler (This is the default, but we don't implement it yet in emscirpten, it prevents the timer signal running before the previous one has finished).
2. Don't run proxied work during a signal handler. Signal handler should always be fast so the main thread should be able to delay running any proxied work until the end of a given signal handler.

Fixes: #20125