[libcxx][test][NFC] Add tests for C++14 constructors of unordered sets

Add tests for C++14 constructors in unordered_set and unordered_multiset:

unordered_[multi]set(size_type sz, const allocator_type& alloc);
unordered_[multi]set(size_type sz, const hasher& hash, const allocator_type& alloc);

template <class InputIt>
unordered_[multi]set(InputIt first, InputIt last, size_type sz, const allocator_type& alloc);

template <class InputIt>
unordered_[multi]set(InputIt first, InputIt last, size_type sz, const hasher& hash, const allocator_type& alloc);

unordered_[multi]set(initializer_list<value_type> init, size_type sz, const allocator_type& alloc);
unordered_[multi]set(initializer_list<value_type> init, size_type sz, const hasher& hash, const allocator_type& alloc);

Reviewed By: ldionne, Quuxplusone, rarutyun, #libc

Differential Revision: https://reviews.llvm.org/D119243