Installer test: Remove fileSystems argument

The config function unintentionally ignored its fileSystems
argument. However, things still worked thanks to the magic of
nixos-generate-config. Yay!
Remove obsolete zsh help text
nixos-generate-config: Use systemd-detect-virt instead of dmidecode

Dmidecode fails in our EFI test with the error "SMBIOS entry point
missing". But we don't need dmidecode because we have already have
systemd-detect-virt.
Merge the EFI test into tests/installer.nix