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

[Drew Adams]

> > Then again, plists should never have existed, in my book.
> > They're just strictly worse than alists as datastructures.
> 
> Indeed. For example, key-value pairs cannot be shared between 
> different plists, and getting the nth key takes twice as long.
> There should be no reason to promote or expand the use of plists.

I happen to agree.  Adding a TEST arg for plist
functions isn't, IMO, promoting or expanding
their use.  It's just recognizing existing use.