Change must-double-locking in 03-practical/21-pfscan_combine_minimal …

…to TODO

On MacOS the mutex type is top because it's zero-initialized global.
On MacOS zero-initialized mutex isn't the same as a default mutex (type 0).

tests/regression/03-practical/21-pfscan_combine_minimal.c

@@ -18,15 +18,15 @@ int pqueue_init(PQUEUE *qp)
 
 void pqueue_close(PQUEUE *qp )
 {
-  pthread_mutex_lock(& qp->mtx); // WARN (must double locking)
+  pthread_mutex_lock(& qp->mtx); // TODO (OSX): WARN (must double locking)
   qp->closed = 1;
   pthread_mutex_unlock(& qp->mtx);
   return;
 }
 
 int pqueue_put(PQUEUE *qp)
 {
-  pthread_mutex_lock(& qp->mtx); // WARN (must double locking)
+  pthread_mutex_lock(& qp->mtx); // TODO (OSX): WARN (must double locking)
   if (qp->closed) {
     // pfscan actually has a bug and is missing the following unlock at early return
     // pthread_mutex_unlock(& qp->mtx);