tests: add integration tests

I was too tired last night (this morning...) I inverted a couple comparisons.

Those simple tests helped isolate the issue.

src/engine/engine.cc

@@ -95,7 +95,6 @@ Engine::Engine(std::shared_ptr<EngineListener> listener)
     : listener_(listener) {}
 
 void Engine::process_orders(std::vector<Order> const& orders) {
-    // FIXME: handle top-of-book changes
     for (auto const& order : orders) {
         std::visit([this](auto const& trade_order) { (*this)(trade_order); },
                    order);