Write tests for nixos-install.

[YellowOnion]

Describe the bug

On several occasions I've had issues where the installer doesn't work on unstable, trying to repair a broken system and have a core package break under this scenario, especially reverting to stable and compile a new custom kernel just to get my system to boot again is really frustrating and a waste of time.

see:
#156121

and:

copying channel...
installing the boot loader...
setting up /etc...
Running in chroot, ignoring request.
Copied "/nix/store/24ljibki63lxk0m11qnw8fh9smh64g3x-systemd-249.7/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/EFI/systemd/systemd-bootx64.efi".
Copied "/nix/store/24ljibki63lxk0m11qnw8fh9smh64g3x-systemd-249.7/lib/systemd/boot/efi/systemd-bootx64.efi" to "/boot/EFI/BOOT/BOOTX64.EFI".
Random seed file /boot/loader/random-seed successfully written (512 bytes).
Created EFI boot entry "Linux Boot Manager".
mktemp: failed to create file via template ‘/mnt/tmp.p7wN2peYyL/tmp.XXXXXXXXXX’: No such file or directory
Traceback (most recent call last):
  File "/nix/store/g66xbwwxlxbxnfq6i1zzdc2nbqydsw28-systemd-boot", line 305, in <module>
    main()
  File "/nix/store/g66xbwwxlxbxnfq6i1zzdc2nbqydsw28-systemd-boot", line 293, in main
    subprocess.check_call("/nix/store/si4frm8y336b00animvm77mkmqmc3k9d-copy-extra-files")
  File "/nix/store/afi0ysqw20yiiw2gr2d28dx40bc4ddf8-python3-3.9.10/lib/python3.9/subprocess.py", line 373, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '/nix/store/si4frm8y336b00animvm77mkmqmc3k9d-copy-extra-files' returned non-zero exit status 1.

[roberth]

We have tests for NixOS installation, in nixos/tests/installer.nix.
All except grub1 (legacy?) currently pass on hydra (search nixos.tests.installer)
You can't write a test without resolving your issue, and you shouldn't resolve your issue without a test, so I don't see the purpose of this issue.

I also want more and better tests, but I don't think this issue is going to help with that.

Closing as duplicate of #156212