Here is the *Messages* buffer after the first part

For information about GNU Emacs and the GNU system, type C-h C-a.

#s(wallpaper-setter "XFCE" "xfconf-query" wallpaper-xfce-command-args #[0 "! \0! \0 " [getenv "DESKTOP_SESSION" ("xubuntu" "ubuntustudio") "XFCE" xdg-current-desktop] 2] nil nil)

The second part still fails

Emacs/emacs-29.0.90/test
✦ ❯ make lisp/image/wallpaper-tests
make[1]: Entering directory '/home/tim/Emacs/emacs-29.0.90/test'
  GEN      lisp/image/wallpaper-tests.log
Running 20 tests (2023-04-11 09:20:51-0400, selector `(not (tag :unstable))')
Test wallpaper--find-command-args/return-list backtrace:
  signal(ert-test-failed (((should (or (not (wallpaper--find-command-a
  ert-fail(((should (or (not (wallpaper--find-command-args)) (listp (w
  (if (unwind-protect (setq value-70 (or (not (wallpaper--find-command
  (let (form-description-71) (if (unwind-protect (setq value-70 (or (n
  (let ((value-70 (gensym "ert-form-evaluation-aborted-"))) (let (form
  (closure (t) nil (let ((value-70 (gensym "ert-form-evaluation-aborte
  ert--run-test-internal(#s(ert--test-execution-info :test #s(ert-test
  ert-run-test(#s(ert-test :name wallpaper--find-command-args/return-l
  ert-run-or-rerun-test(#s(ert--stats :selector (not ...) :tests [...
  ert-run-tests((not (tag :unstable)) #f(compiled-function (event-type
  ert-run-tests-batch((not (tag :unstable)))
  ert-run-tests-batch-and-exit((not (tag :unstable)))
  command-line-1(("-L" ":." "-l" "ert" "-l" "lisp/image/wallpaper-test
  command-line()
  normal-top-level()
Test wallpaper--find-command-args/return-list condition:
    (ert-test-failed
     ((should
       (or
(not ...)
(listp ...)))
      :form
      (or
       (not
(wallpaper--find-command-args))
       (listp
(wallpaper--find-command-args)))
      :value nil))
   FAILED   1/20  wallpaper--find-command-args/return-list (0.000107 sec) at lisp/image/wallpaper-tests.el:128
   passed   2/20  wallpaper--find-command/return-string (0.000047 sec)
   passed   3/20  wallpaper--find-setter (0.000185 sec)
   passed   4/20  wallpaper--find-setter/call-predicate (0.000195 sec)
   passed   5/20  wallpaper--find-setter/set-current-setter (0.000165 sec)
   passed   6/20  wallpaper--format-arg/filename (0.000345 sec)
   passed   7/20  wallpaper--format-arg/filename-hex (0.009361 sec)
   passed   8/20  wallpaper--format-arg/height (0.000067 sec)
   passed   9/20  wallpaper--format-arg/monitor (0.000066 sec)
   passed  10/20  wallpaper--format-arg/screen (0.000065 sec)
   passed  11/20  wallpaper--format-arg/width (0.000068 sec)
   passed  12/20  wallpaper--format-arg/workspace (0.000057 sec)
   passed  13/20  wallpaper--get-default-file/empty-gives-nil (0.006376 sec)
   passed  14/20  wallpaper--get-default-file/file-at-point (0.014131 sec)
   passed  15/20  wallpaper--get-default-file/visiting-file (0.000416 sec)
   passed  16/20  wallpaper--image-file-regexp/return-string (0.000050 sec)
Desktop wallpaper changed to /tmp/emacs-test-CxuJrO.jpg
   passed  17/20  wallpaper-set/calls-init-action (0.001059 sec)
   passed  18/20  wallpaper-set/calls-wallpaper-set-function (0.000172 sec)
Desktop wallpaper changed to /tmp/emacs-test-l6ctHj.jpg
   passed  19/20  wallpaper-set/runs-command (0.001644 sec)
   passed  20/20  wallpaper-set/runs-command/detach (0.001639 sec)

Ran 20 tests, 19 results as expected, 1 unexpected (2023-04-11 09:20:51-0400, 0.084415 sec)

1 unexpected results:
   FAILED  wallpaper--find-command-args/return-list

make[1]: *** [Makefile:181: lisp/image/wallpaper-tests.log] Error 1
make[1]: Leaving directory '/home/tim/Emacs/emacs-29.0.90/test'
make: *** [Makefile:247: lisp/image/wallpaper-tests] Error 2