Improved code [skip ci]

Author: ankane

test/helper.hpp

@@ -17,7 +17,7 @@ void assert_equal(const T& left, const U& right, const std::source_location& loc
 }
 
 template<typename T>
-void assert_exception(std::function<void(void)> code, std::optional<std::string_view> message = std::nullopt) {
+void assert_exception(const std::function<void(void)>& code, std::optional<std::string_view> message = std::nullopt) {
     std::optional<T> exception;
     try {
         code();