Let's allow configuring the debug tty independently of enabling/disabling the debug shell. This allows mkosi to configure the correct tty while leaving enabling/disabling the debug tty to the user.
Let's make sure the units generated by generators are generated at the right stage.
Add some explicit tests for various generators we ship, e.g.: - systemd-debug-generator - systemd-environment-d-generator - systemd-fstab-generator