Yes, that does it!
❯ make lisp/image/wallpaper-tests
make[1]: Entering directory '/home/tim/Emacs/emacs-29.0.90/test'
ELC+ELN lisp/image/wallpaper-tests.elc
GEN lisp/image/wallpaper-tests.log
Running 20 tests (2023-04-11 09:55:36-0400, selector `(not (tag :unstable))')
passed 1/20 wallpaper--find-command-args/return-list (0.021983 sec)
passed 2/20 wallpaper--find-command/return-string (0.000102 sec)
passed 3/20 wallpaper--find-setter (0.000444 sec)
passed 4/20 wallpaper--find-setter/call-predicate (0.000424 sec)
passed 5/20 wallpaper--find-setter/set-current-setter (0.000388 sec)
passed 6/20 wallpaper--format-arg/filename (0.000593 sec)
passed 7/20 wallpaper--format-arg/filename-hex (0.010103 sec)
passed 8/20 wallpaper--format-arg/height (0.000071 sec)
passed 9/20 wallpaper--format-arg/monitor (0.000067 sec)
passed 10/20 wallpaper--format-arg/screen (0.000062 sec)
passed 11/20 wallpaper--format-arg/width (0.000059 sec)
passed 12/20 wallpaper--format-arg/workspace (0.000054 sec)
passed 13/20 wallpaper--get-default-file/empty-gives-nil (0.006273 sec)
passed 14/20 wallpaper--get-default-file/file-at-point (0.002885 sec)
passed 15/20 wallpaper--get-default-file/visiting-file (0.000414 sec)
passed 16/20 wallpaper--image-file-regexp/return-string (0.000057 sec)
Desktop wallpaper changed to /tmp/emacs-test-3JWhD9.jpg
passed 17/20 wallpaper-set/calls-init-action (0.001792 sec)
passed 18/20 wallpaper-set/calls-wallpaper-set-function (0.000192 sec)
Desktop wallpaper changed to /tmp/emacs-test-s7gOm7.jpg
passed 19/20 wallpaper-set/runs-command (0.001794 sec)
passed 20/20 wallpaper-set/runs-command/detach (0.001800 sec)
Ran 20 tests, 20 results as expected, 0 unexpected (2023-04-11 09:55:36-0400, 0.050690 sec)
make[1]: Leaving directory '/home/tim/Emacs/emacs-29.0.90/test'