nixos/tests: Add a test for boot stage 1

We already have a small regression test for #15226 within the swraid
installer test. Unfortunately, we only check there whether the md
kthread got signalled but not whether other rampaging processes are
still alive that *should* have been killed.

So in order to do this we provide multiple canary processes which are
checked after the system has booted up:

 * canary1: It's a simple forking daemon which just sleeps until it's
            going to be killed. Of course we expect this process to not
            be alive anymore after boot up.
 * canary2: Similar to canary1, but tries to mimick a kthread to make
            sure that it's going to be properly killed at the end of
            stage 1.
 * canary3: Like canary2, but this time using a @ in front of its
            command name to actually prevent it from being killed.
 * kcanary: This one is a real kthread and it runs until killed, which
            shouldn't be the case.

Tested with and without 67223ee2

 and everything works as expected, at
least on my machine.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>