Test and document relations between schedule and metrics file_count_aim, file_age_aim_seconds

At the moment it is difficult to understand how the metrics for `file_count_aim` and `file_age_aim` are calculated:
- As a reference, the cron job definition of `schedule`'s configuration parameter is used.
- `FindPrevious` in https://github.com/dreitier/backmon/blob/main/backup/cron.go is used for finding (?) all matching files before the next or previous schedule and then updates the metrics accordingly.

1. Add tests for `FindPrevious` to check if it's output is expected
2. Add more detailled documentation for behavior and calculation of `file_count_aim` and `file_age_aim`.