log-levels concept exercise

[colinleach]

This one feels messy and unsatisfactory, but for now it's the best I've got. Because of all the Docker-related difficulties in adding tidyverse to the test runner earlier this year, I've stuck to core R functions while dreaming of stringr.

A number of other concepts have this as a prerequisite, but all most of them really need is paste() and/or sprintf().

Any thoughts on the best way forward?