nixos/test-driver: allow easier sharing between expression and driver script

[Synthetica9]

Currently, we use a lot of string interpolation to get information from the nix expression defining the test to the script running the driver. It would be nice to have a more principled way of doing this. My thought is to have a field in the driver that is written by Nix, converted to JSON, and then read by the driver script.

A minimal example of how this would work:

import ./make-test-python.nix ({ ... } : {
  machine = { ... }: { };

  shared = {
    foo = 3;
  };

  testScript =
    ''
      assert shared.foo == 3
    '';
})