[libcxx] Add missing <cstddef> includes in tests

Some tests use type std::max_align_t, but don't include <cstddef> header
directly. As a result, these tests won't compile against some conformant
libraries.

Reviewed as https://reviews.llvm.org/D54645.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 347232