[TSan] Make test fail more predictably

[yln]

Summary

This test would hang when the system ran out of resources and we fail to
create all 300 threads.

compiler-rt/test/tsan/lots_of_threads.c

int main() {
   const int kThreads = 300;
   const int kIters = 10;
   barrier_init(&barrier, kThreads + 1);
   pthread_attr_t attr;
   pthread_attr_init(&attr);
   pthread_attr_setstacksize(&attr, 16 << 20);
   for (int iter = 0; iter < kIters; iter++) {
     pthread_t threads[kThreads];
-    for (int t = 0; t < kThreads; t++)
-      pthread_create(&threads[t], &attr, thr, 0);
+    for (int t = 0; t < kThreads; t++) {
+      int err = pthread_create(&threads[t], &attr, thr, 0);
+      if (err) {
+        fprintf(stderr, "Failed to create thread #%d\n", t);
+        return 1;
+      }
+    }
     barrier_wait(&barrier);
     for (int t = 0; t < kThreads; t++)
       pthread_join(threads[t], 0);
   }
   pthread_attr_destroy(&attr);
   fprintf(stderr, "DONE\n");
   return 0;
 }
 
 // CHECK: DONE