[TSan][libdispatch] Fix test

[yln]

Summary

Would this simpler test do the trick as well?

[kubamracek]

I don't think this is testing the same thing. Previously, we use barrier_wait to enforce a situation we want to test.

compiler-rt/test/tsan/Darwin/gcd-barrier.mm

 // RUN: %clang_tsan %s -o %t -framework Foundation
 // RUN: %run %t 2>&1 | FileCheck %s
 
 #import <Foundation/Foundation.h>
 
 #import "../test.h"
 
 long global;
 
 int main() {
   fprintf(stderr, "Hello world.\n");
   print_address("addr=", 1, &global);
   barrier_init(&barrier, 2);
 
   dispatch_queue_t q = dispatch_queue_create("my.queue", DISPATCH_QUEUE_CONCURRENT);
-  dispatch_queue_t bgq = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
 
-  dispatch_async(bgq, ^{
-    dispatch_sync(q, ^{
-      global = 42;
-    });
-    barrier_wait(&barrier);
-  });
-
-  dispatch_async(bgq, ^{
-    barrier_wait(&barrier);
-    dispatch_barrier_sync(q, ^{
-      global = 43;
-    });
-
-    dispatch_async(bgq, ^{
-      barrier_wait(&barrier);
-      global = 44;
-    });
+  dispatch_async(q, ^{
+  dispatch_barrier_async(q, ^{
+  dispatch_async(q, ^{
 
     barrier_wait(&barrier);
-    
-    dispatch_sync(dispatch_get_main_queue(), ^{
-      CFRunLoopStop(CFRunLoopGetCurrent());
-    });
   });
 
-  CFRunLoopRun();
+  barrier_wait(&barrier);
   fprintf(stderr, "Done.\n");
 }
 
 // CHECK: Hello world.
 // CHECK: Done.
 // CHECK-NOT: WARNING: ThreadSanitizer