[Gzz] C++ unit tests

[Tuomas Lukka]

I've been looking at this now in detail: cxxtest is otherwise nice, but lacks
one major feature we need: we have several cases where we have an interface Foo,
and several implementations (Bar, Baz). The interface is required to adhere to
some contract, e.g. Foo.x(Foo.y(s)) = s.

What we really want to do is to make one test for the interface and inherit from
it. This is not possible in cxxtest, but is (through templates, it seems) in 
cppunit.

        Tuomas