fixed backend test compilation

include/matador/sql/connection_pool.hpp

@@ -81,7 +81,8 @@ public:
       connection_repo_.emplace_back(count, info_);
       auto &conn = connection_repo_.back();
       idle_connections_.emplace(conn.first, &conn);
-      conn.second.open();
+      // Todo: handle result
+      std::ignore = conn.second.open();
       --count;
     }
   }