Use LIBCPP_STATIC_ASSERT in chrono test

[mordante]

          As the `is_steady` is tested only once (ditto `tai_clock::is_steady`) and doesn't seem necessary for twice, should we just remove it and write `LIBCPP_STATIC_ASSERT(std::chrono::gps_clock::is_steady == false);` instead?

Originally posted by @frederick-vs-ja in #132532 (comment)