Use cli pluralization in print method for `action_levels()`

[hfrick]

It'd be nice if the message said "1 test unit". Interestingly, it's also missing a space here, while it does show up alright on my console 🤔

library(pointblank)

action_levels(warn_at = 1)
#> -- The `action_levels` settings
#> WARN failure threshold of 1test units.
#> ----

^{Created on 2025-07-09 with reprex v2.1.1}