Fixing test where return by reference would cause unexpected results when compiled with release flags

tests/unit_dag.cpp

@@ -69,7 +69,7 @@ TEST_CASE("DAG Traversal Tests", "[dag]") {
         std::vector<int> visitOrder(N_VERTICES);
         size_t i = 0;
         while (!dag.allVisited()) {
-            const auto &v = dag.visitNext().value();
+            const auto v = dag.visitNext().value();
             dag.completeVisit(v.first);
             visitOrder[v.first] = i;
             ++i;