[libcxx][test][NFC] Add tests for constructors of unordered [multi]map since C++14

Add tests for C++14 constructors in unordered_map and unordered_multimap

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

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

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

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

Reviewed By: ldionne, rarutyun, #libc

Differential Revision: https://reviews.llvm.org/D119236