Fix data race in ordered_queue tests

Was caused accessing the global variable *num_items* in the constructor
of an Item sent to the queue. This is not permissible since items are accessed
in parallel by separate threads.

testing/unit_tests/ordered_queue.cpp

@@ -22,7 +22,7 @@
 using namespace MR;
 using namespace App;
 
-size_t num_items;
+std::atomic<size_t> num_items;
 
 #define START(msg)                                                                                                     \
   {                                                                                                                    \