Use an ostringstream for the output and make the test actually confirm the results.

game_engine.hpp

@@ -2,14 +2,16 @@
 #include <map>
 #include <vector>
 #include <array>
+#include <sstream>
 
 using namespace std;
 
 class Brainfucker {
-  size_t dp = 0;
-  size_t ip = 0;
 
   public:
+  size_t dp = 0;
+  size_t ip = 0;
+  ostringstream out;
   array<int, 100> data = {};
   string code = {};