One convenient way to run tests in development is to evaluate

    (setq #+sbcl sb-rt:*do-tests-when-defined*
	  #-sbcl regression-test:*do-tests-when-defined*
	  t)

and then use SLIME's C-c C-c to run individual tests and its C-c C-k to run
all tests in the current file.  You'll get REPL output if any fail.