[test] Fix data race in samples/ffi/async/async_test.

TEST=tsan
Bug: #55615
Change-Id: I4de5701037640f5b0ae6b176f695907c4c5e1ef5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365209
Commit-Queue: Daco Harkes <[email protected]>
Reviewed-by: Daco Harkes <[email protected]>

Author: rmacnak-google

runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc

@@ -480,9 +480,10 @@ intptr_t MyCallbackBlocking(intptr_t a) {
   auto callback = my_callback_blocking_fp_;  // Define storage duration.
   std::condition_variable cv;
   bool notified = false;
-  const Work work = [a, &result, callback, &cv, &notified]() {
+  const Work work = [a, &result, callback, &mutex, &cv, &notified]() {
     result = callback(a);
     printf("C Da:     Notify result ready.\n");
+    std::unique_lock<std::mutex> lock(mutex);
     notified = true;
     cv.notify_one();
   };