tsan: improve lots_of_threads test

The current 10 threads is not particularly "lots" and not stressful.
Create 10x300 threads and ensure they all are running at the same time.

Depends on D106953.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D106954

compiler-rt/test/tsan/lots_of_threads.c

@@ -10,18 +10,21 @@ void *thr(void *arg) {
 }
 
 int main() {
-  const int kThreads = 10;
+  const int kThreads = 300;
+  const int kIters = 10;
   barrier_init(&barrier, kThreads + 1);
-  pthread_t t[kThreads];
   pthread_attr_t attr;
   pthread_attr_init(&attr);
   pthread_attr_setstacksize(&attr, 16 << 20);
-  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
-  for (int i = 0; i < kThreads; i++)
-    pthread_create(&t[i], &attr, thr, 0);
+  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);
+    barrier_wait(&barrier);
+    for (int t = 0; t < kThreads; t++)
+      pthread_join(threads[t], 0);
+  }
   pthread_attr_destroy(&attr);
-  barrier_wait(&barrier);
-  sleep(1);
   fprintf(stderr, "DONE\n");
   return 0;
 }