These had been waiting on the ability to use `std::copy` from constexpr code (which in turn had been waiting on the ability to use `is_constant_evaluated()` to switch between `memmove` and non-`memmove` implementations of `std::copy`). That work landed a while ago, so these algorithms can all be constexpr in C++20 now.

While I'm in the area, update the tests. (This part has scope-crept a ton.)

According to my research, here are the remaining algorithms that P0202+P0879 marked "constexpr," which are still not implemented as "constexpr" by libc++. We'd have to implement these before we could advertise the `__cpp_lib_constexpr_algorithms` feature-test macro:

reverse partition sort nth_element next_permutation prev_permutation push_heap pop_heap make_heap sort_heap partial_sort partial_sort_copy