bug#47425: 26.3; `plist-get', `plist-put' should accept a TEST function

[Basil L. Contovounesios]

(let ((plist (list :a t)))
  (setf (plist-get plist :a) nil)
  plist)
;; => (:a nil)

(let ((plist (list :a t)))
  (setf (plist-get plist :a #'eq) nil)
  plist)
;; error-> wrong-number-of-arguments

Do we want the expander in gv.el to understand the new argument?

There's also the question of map.el.  Custom predicates are deprecated
there, since they rely on knowing the underlying map type.  Do we want
plist operations to understand custom predicates regardless?

Thanks,

-- 
Basil