nixos/tests: add test for showing child configuration in grub menu

- Create a child configuration named "Work" with an extra config file.
- Name the default configuration as "Home" :-)
- Once the VM is setup, reboot and verify that it has booted into
default configuration.
- Reboot into the "Work" configuration via grub.
- Verify that we have booted into the "Work" configuration and that
the extra config file is present.

This test works for the simple grub configuration and simple UEFI
Grub configuration. UEFI Systemd is not included in the test.