Fix browser.test_fetch_stream_abort_asan race condition

[juj]

Fix browser.test_fetch_stream_abort_asan race condition by avoiding double free of the fetch structure.

The issue in the test was that onprogress handler in the test would queue a setimmediate to free the fetch structure after the turn of the event loop.

But the browser might also have fed a second onprogress handler call before the previous setimmediate, which would also trigger a second setimmediate callback.

Then the two setimmediate callbacks would fire, resulting in a double emscripten_fetch_close() calls, and the asan test croaking:

Fix the race condition by ensuring only a single abort is queued.

[sbc100]

How about:

if (!fetch_abort_queued) {
   fetch_abort_queued = true;
   emscripten_set_immediate([](void *arg) {
      ...
   }
}

[juj]

I'm not sure that changes anything for the better - deeper nesting and testing for negation.