[WIP/RFC] Run vroom integration tests in CMake.

When running tests locally, developers have to take care to have the latest vroom and neovim-python installed. This PR moves the installation of these dependencies into CMake, like it was done with busted etc.

To be able download the latest versions of these two dependencies, the CMake download code was modified to accept an md5 sum of `SKIP`.

I'm not sure if this *should* be done (especially the `PYTHONPATH` stuff is very hacky, and I don't know if this can be improved), but I'd like to hear your opinions.