default.nix: Run test cases for C++ as well.
default.nix: Run test cases for C++ as well.

Running Neko tests is quite fast, because we don't need to compile
anything, but if we want to make sure everything works like expected we
need to run those tests also for C++.

That's why I left test.hxml as-is, because it's meant to do a quick run
of the test cases.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

aszlig/hase