C++ duchain unit tests

[David Nolden]

I guess the main problem is that the old contexts either aren't
removed from the duchain at the end of the tests, or the
remove(topContext) doesn't work properly. This needs to be found out.

The first problem could be solved by using something similar to my
recently added "InsertIntoDUChain" class, which automatically releases
the top-context in the destructor.

Greetings, David