NixOS VM tests

The test infrastructure entry point is nixos/lib/testing.nix. The test infra relies on the qemu build-vm code to generate virtual machines.

It will generate a test driver (a wrapper of nixos/lib/test-driver/test-driver.pl) in charge of creating the network. It will start one vde-switch and its associated socket per vlan (defined in virtualisation.vlans). IPs are assigned declaratively according to the number of vlan via the function `assignIPAddresses`.

The driver (of the form /nix/store/668bqxvsv6rn9hy8n4nmaps9ma2i5k4r-nixos-test-driver-<TESTNAME>) will launch the different vms passed as arguments. The wrapper `bin/nixos-run-vms` is in charge to start the driver with the correct VM script as arguments.

Once the driver is loaded, depending on the environment variables `tests` it will run in an interactive mode or run some perl code (`testScript`). In interactive mode, you can run `startAll` followed by `joinAll` to start and keep the VM alive

How to debug tests ?

You can run the tests interactively as described in nixos manual. When you run `nix-build ./nixos/tests/login.nix`, the resulting output gives you a summary of the results, but to gain access to the VM, you can run `nix repl ./nixos/tests/login.nix` and see the ran VM via `driver.outPath`.