[TSan][libdispatch] Change test to have two simultaneous timers

[yln]

Summary

Change test semantics by waiting for both timer callbacks at the end
instead of serializing operations: start/wait timer 1 then 2.

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

int main(int argc, const char *argv[]) {
   dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
 
   my_global = 10;
   dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_MSEC)), q, ^{
     my_global = 42;
 
     dispatch_semaphore_signal(done);
   });
-  dispatch_semaphore_wait(done, DISPATCH_TIME_FOREVER);
 
   my_global2 = 10;
   dispatch_after_f(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_MSEC)), q, NULL, &callback);
 
   dispatch_semaphore_wait(done, DISPATCH_TIME_FOREVER);
+  dispatch_semaphore_wait(done, DISPATCH_TIME_FOREVER);
   fprintf(stderr, "done\n");
   return 0;
 }
 
 // CHECK: start
 // CHECK: done
 // CHECK-NOT: WARNING: ThreadSanitizer