The free, industry standard test suite you can use is the TAHI test sutie, which can be used to get an IPv6 Ready Logo for your device (mentioned by Martin in another answer).
The TAHI suite runs on FreeBSD and will test for basic functionality of your IPv6 stack. You will still need to test real-world use cases, like actually using the device over IPv6 rather than IPv4. ;-) (That is, having a working stack does not guarantee that your real-world use cases will work; someone may have erroneously opened an IPv4-only socket, or done something else in the code which is not IPv6-friendly.)