4b128008c5d9fde881ce1b0a25e60ae0415a14d5 in nixpkgs introduced a default
hashedPasswordFile for root in NixOS tests, which takes precedence over
the password option set in the nix-copy test.
Change-Id: Iffaebec5992e50614b854033f0d14312c8d275b5
without these changes the tests will very repeatably (although not very
reliably) wedge in our runs. the ssh command starts, opens a sessions,
does something, the session closes again, but the test does not move on.
adding *just* the redirect and not the unit waits is not sufficient
either, it needs both. this feels like a bug in the nixos testing
framework somewhere, but digging that far is not in the cards right now.
Change-Id: Idab577b83a36cc4899bb5ffbb3d9adc04e83e51c
