I've had a few questions about why I use my own little testing framework, saru, for testing, rather than using something standard like cxxunit or whatever.
One of the key problems is testing across multiple languages. We use C++, PHP and python for various pieces of our pipeline. Saru was designed so that plugging in a new language is easy. So if someday we need to support Java then I'm not worried. Now we could use cxxunit for C++, phpunit for php, etc, but I like to have unified reporting and a bit more integration of these tests.
Another problem I have with using a pure C++ framework is sometimes I want to test the condition "Class Foo should not be default constructible." The easiest way for me to check that is to have a fragment of code that should fail to compile with given error messages. You can't test that with a C++ testing framework. (In this particular case you may be able to do something with SFINAE style template hackery, or fork and call a compiler, but these just feel very hackish)
My third reason for using saru is that sometimes I want to do things that are not easily expressed in C++, but easily expressed in other languages. Things like "are the source files correctly formatted?", or "do all C++ exceptions have a corresponding PHP handler?", or "are there tests for each of the classes in this directory?". These are all C++ related questions, but much easier expressed in bash using awk/sed/grep or python than in pure C++.
So thats why I use saru.