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

game_engine.cpp

@@ -121,7 +121,7 @@ void Brainfucker::run(int ticks) {
         data[dp] = data[dp] - 1;
         break;
       case '.':
-        cout << (char)data.at(dp);
+        out << (char)data.at(dp);
         break;
       case ',':
         print(ERROR, "Not implemented.\n");
@@ -162,7 +162,6 @@ void Brainfucker::reset() {
   data.fill(0);
 }
 
-
 GameEngine::GameEngine(int hp) : hit_points(hp) {};
 
 int GameEngine::determine_damage(string &type) {

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 = {};
 

tests/game_engine.cpp

@@ -1,5 +1,8 @@
 #include <catch2/catch_test_macros.hpp>
 #include "../game_engine.hpp"
+#include <fmt/core.h>
+
+using namespace fmt;
 
 TEST_CASE("basic brainfuck test", "[brainfuck]") {
   Brainfucker bf;
@@ -26,11 +29,16 @@ TEST_CASE("basic brainfuck test", "[brainfuck]") {
 
 TEST_CASE("brainfuck loop test", "[brainfuck]") {
   Brainfucker bf;
+  const string expected{"Hello World!\n"};
   // this is a hello world program from wikipedia
+  // but at the end I rewind dp so I can analyze it
   string code{"++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++."};
 
   bf.set_code(code);
   bf.run(code.size());
+
+  string output = bf.out.str();
+  REQUIRE(output == expected);
 }
 
 TEST_CASE("game engine can start and take hit", "[brainfuck]") {