test(SYSTEMD): make the man command succeed

[LaszloGombos]

Document the reason of the existing workaround.

Move the existing logic to the initramfs.testing generation phase and make the steps more generic.

[stale[bot]]

This issue is being marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. If this is still an issue in the latest release of Dracut and you would like to keep it open please comment on this issue within the next 7 days. Thank you for your contributions.

[LaszloGombos]

not stale