Eliminate 'CORD_*printf is never used' cppcheck style warnings (cordtest)

Eliminate 'CORD_*printf is never used' cppcheck style warnings (cordtest)

Minimal testing of CORD_[v][f]printf is added to cordtest.

* cord/tests/cordtest.c: Include stdarg.h.
* cord/tests/cordtest.c: Reformat the comment describing cordtest.
* cord/tests/cordtest.c (wrap_vprintf, wrap_vfprintf): New function
(calling CORD_v[f]printf).
* cord/tests/cordtest.c (test_printf): Call CORD_printf, wrap_vfprintf,
wrap_vprintf for CORD_EMPTY (with the output to stdout); add TODO item.