This not-so-frequently asked question is primarily for ASDF developers, but those who encounter an unexpected error in some test may be interested, too.
Here's the procedure for experimenting with tests in a REPL:
;; BEWARE! Some tests expect you to be in the .../asdf/test directory
;; If your REPL is not there yet, change your current directory:
;; under SLIME, you may: ,change-directory ~/common-lisp/asdf/test/
;; otherwise you may evaluate something like:
(require "asdf") (asdf:upgrade-asdf) ;load UIOP & update asdf.lisp
(uiop:chdir (asdf:system-relative-pathname :asdf "test/"))
(setf *default-pathname-defaults* (uiop:getcwd))
;; Load the test script support.
(load "script-support.lisp")
;; Initialize the script support.
;; This will also change your *package* to asdf-test.
;; NB: this function is also available from package cl-user,
;; and also available with the shorter name da in both packages.
(asdf-test::debug-asdf)
;; In case you modified ASDF since you last tested it,
;; you need to update asdf.lisp itself by evaluating 'make' in a shell,
;; or (require "asdf") (asdf:load-system :asdf) in another CL REPL,
;; if not done in this REPL above.
;; *Then*, in this REPL, you need to evaluate:
;(asdf-test::compile-load-asdf)
;; Now, you may experiment with test code from a .script file.
;; See the instructions given at the end of your failing test
;; to identify which form is needed, e.g.
(frob-packages)
(asdf::with-asdf-cache () (load "test-utilities.script"))