The thing to realise is that, at least is my experience, REPL based development doesn't mean your are actually typing code into the REPL.

Instead, you develop code in a file, but constantly evaluate code as you go along.

When I work on a clojure project, I very rarely open the actual REPL, but I am constantly evaluating code and experimenting with different implementations of functions.

Then, when I'm happy with the results, I ask the editor to evaluate and insert the results back into the editor. This then becomes the unit test.