bug#46722: test-map-into fails

[dick]

For unclear reasons, the hash-table `cl--generic-dispatchers` keyed off
raw, unburnished defstructs returns values for non-existent keys.

I normalized its key to a signature string, and,

for i in {1..300} ; do src/emacs -Q -batch -f batch-byte-compile foo.el
; src/emacs -Q -batch -l foo.elc ;  done ;

where foo.el is (map-into nil '(hash-table)) ran without incident.