Fix browser.test_fetch_stream_abort_asan race condition (#25931)

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:

<img width="1361" height="1352" alt="image"
src="https://github.com/user-attachments/assets/067491b5-0093-4072-98a6-aa02911e3693"
/>

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

Author: juj

test/fetch/test_fetch_stream_abort.cpp

@@ -11,6 +11,8 @@
 #include <emscripten/fetch.h>
 #include <emscripten/eventloop.h>
 
+bool fetch_abort_queued = false;
+
 int main() {
   emscripten_fetch_attr_t attr;
   emscripten_fetch_attr_init(&attr);
@@ -32,6 +34,8 @@ int main() {
       fetch->dataOffset,
       fetch->dataOffset + fetch->numBytes);
 
+    if (fetch_abort_queued) return;
+    fetch_abort_queued = true;
     emscripten_set_immediate([](void *arg) {
       emscripten_fetch_t *fetch = (emscripten_fetch_t *)arg;
       printf("Abort fetch when downloading\n");