unit test needed for when somnus is under 'NGINX Unit' mode

[Thesephi]

a unit test for when somnus is under 'NGINX Unit' mode would be nice (hint: it'll throw if NXT_UNIT_INIT is mocked, or we can consider rolling up a Docker instance to test under a real NGINX Unit environment?)

Originally posted by @Thesephi in #6 (comment)