This commit adds a new NixOS VM test, which tests that `nix upgrade-nix`
works on both kinds of profiles (manifest.nix and manifest.json).
Done as a separate commit from 831d18a13, since it relies on the
--store-path argument from 026c90e5f as well.
Change-Id: I5fc94b751d252862cb6cffb541a4c072faad9f3b
