I'm fond of single-file libraries, and like to embed test cases within
the libraries, with a simple way of disabling the test suite when the
library is deployed (to free up some resources).

Do you see SRFI-78 as addressing those needs?

I've been using Testeez ("http://www.neilvandyke.org/testeez/") and the
edit-the-source-file kludge for enabling/disabling the tests.

A related question is how SRFI-78 works in the context of a forthcoming
standard module system.  I think most of the design work for a unit
testing framework is in addressing practical usage questions like these.