[list helpers] test: added test::non_copyable_thing default constru…

…ctor

With GCC-14.2.1, compiling with `-std=c++23`: the expression `T{}` seems to expect an explicit default constructor
if T (only) has deleted copy-constructors and defaulted move-constructors.

tests/list_helpers.cpp

@@ -38,6 +38,8 @@ namespace test
 {
     struct non_copyable_thing
     {
+        non_copyable_thing() = default;
+
         non_copyable_thing(const non_copyable_thing&) = delete;
         non_copyable_thing& operator = (const non_copyable_thing&) = delete;