[TSan][libdispatch] Fix test

[yln]

Summary

We have barrier_init(..., 2), but 4 calls to barrier_wait.

I think in this test we want to show that dispatch_barrier synchronizes
with all blocks before and after on the same queue. So we need to use
the same queue (bgq -> q).

[kubamracek]

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

31

How is this a problem? barrier_wait can be used multiple times and it will only trigger for every N times it's waited

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

int main() {
   });
 
   dispatch_async(bgq, ^{
     barrier_wait(&barrier);
     dispatch_barrier_sync(q, ^{
       global = 43;
     });
 
-    dispatch_async(bgq, ^{
+    barrier_init(&barrier, 2);
+    dispatch_async(q, ^{
       barrier_wait(&barrier);
       global = 44;
     });
 
     barrier_wait(&barrier);
     
     dispatch_sync(dispatch_get_main_queue(), ^{
       CFRunLoopStop(CFRunLoopGetCurrent());