wchar: Add more C++ tests. Reported by Christian Biesinger <cbiesinger@google.com> in <https://lists.gnu.org/archive/html/bug-gnulib/2019-12/msg00009.html>. * tests/test-wchar-c++3.cc: New file. * modules/wchar-c++-tests (Files): Add it. (Makefile.am): Compile it.