Moved the game engine into its own unit and then made a failing test for it.

tests/game_engine.cpp

@@ -0,0 +1,10 @@
+#include <doctest.h>
+#include "../game_engine.hpp"
+
+TEST_CASE("game engine can start and take hit") {
+  // test fails on purpose right now
+  GameEngine game{100};
+  string err{"error"};
+  game.hit(err);
+  CHECK(game.is_dead() == true);
+}

tests/test1.cpp

@@ -1,5 +1,5 @@
 #define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
-#include "doctest.h"
+#include <doctest.h>
 
 int factorial(int number) { return number <= 1 ? number : factorial(number - 1) * number; }