test-driver.py: Use /tmp/shared for copy_from_vm
test-driver.py: Use /tmp/shared for copy_from_vm

The docstring says it uses a directory shared among all vms, although
that doesn't seem necessary for the functionality. However, it does need
to be consistent between the guest and host.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs