bug#36430: mcron would benefit from a better way to test jobs

[Robert Vollmert <rob@vllmrt.net>]

My issue:

Defined a mcron job in config.scm scheduled to run once a day,
with a scheme expression. How do I test this?

herd schedule mcron lists the job as merely a “Lambda expression”.
I learned how to give it a descriptive name, but still there’s
no script linked that I can run by hand.

One major improvement would be to have:

1. `herd schedule mcron` lists jobs with some kind of id
2. `herd execute mcron <id>` runs the specific mcron job

Or perhaps, any mcron job could be turned into a simple argument-less
guile or shell script that’s shown in the schedule listing?

Thoughts?