By some miracle, before, it was possible to reconnect to the `node1` without
doing any relevant dance.
But now we are direct booting (¿), it seems like we need to do the right things.
This introduces a `check_output` flag for `execute` because we do not want to steal the
messages from the backdoor service as we might execute the kexec too fast compared
to when we will reconnect.
Therefore, we will let the message in the pipe if needed.
There's no reason to use a bootloader when testing kexec, this is a feature
that reboots *directly* in the kernel, if anything, we should just direct boot the
kernel and reboots in the kernel.
A bootloader test really makes sense to test "default" systemctl kexec behavior which is already broken
because systemctl kexec will read the ESP to determine what to kexec by default.
`nixos/modules/installer/kexec/kexec-boot.nix` doesn't contain any
custom NixOS config, other than importing `netboot-minimal.nix` (which
imports `netboot-base.nix`, which imports `netboot.nix`.
`netboot.nix` really is just describing a self-contained system config,
running entirely off kernel and initrd, so we might as well move the
kexec script generation there as well.
`netboot.nix` already contains some `system.build` attributes.
Provide a `system.build.kexecTree` attribute (and `kexecScript` for
composability).
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
