I doubt that you will find it under Posix. Just to clear misinformation posix is actually standard. Best bet would be to get certification for programing on posix compliant OS like Linux, Solaris ...
Whole concept of certification is rather problematic. It is made to extort some money, not to teach. Only one which I know of and is related to C++ is Symbian certified developer.
As usual with C++ self-study is the only way, download few books and study.
If whole idea is related to job search once when you are done you can do on brainbench.com free C++ test and show that to potential employer.