Regression tests are absolutely awesome, especially if written for nontrivial components which will later be modified or expanded.
Having worked with dynamic languages quite a bit, I found that iff your language has a decent REPL, so you can code iteratively, you don't need unit tests to drive your design. You perform those tests in REPL yourself, which helps you flesh out the design, but you don't have layers of code that'll make changing the design harder.
The way I personally work with Lisp is a mix of writing code in a file, compiling particular parts of it and playing around with it in REPL until I've figured out the right design - at which point I may as well start adding some tests around tricky places. The idea is to not burden yourself with permanent tests until you have your design figured out.
Having worked with dynamic languages quite a bit, I found that iff your language has a decent REPL, so you can code iteratively, you don't need unit tests to drive your design. You perform those tests in REPL yourself, which helps you flesh out the design, but you don't have layers of code that'll make changing the design harder.
The way I personally work with Lisp is a mix of writing code in a file, compiling particular parts of it and playing around with it in REPL until I've figured out the right design - at which point I may as well start adding some tests around tricky places. The idea is to not burden yourself with permanent tests until you have your design figured out.