test/algorithm/sort.hpp defines compare_func but never uses it

[Quuxplusone]

https://github.com/kvasir-io/mpl/blob/development/test/algorithm/sort.hpp#L16-L17

template <typename E1, typename E2>
using compare_func = bool_<(E1::value < E2::value)>;

but compare_func is never used in the test case that follows. It's not obvious how it would be passed to mpl[::eager?]::sort.

Also, ideally,

using empty_sort_test = eager::sort<list<>>;

would be followed by

static_assert(std::is_same_v<empty_sort_test, list<>>);