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

[Drew Adams]

> > I'd be curious to hear of a use case where plists are better than
> > alists while at the same time requiring a non-eq comparison.
> 
> They're equivalent in features, yes, but people prefer plists for
> simple structures because they look nicer.  I'd say they've proven
> their popularity over half a century.

Yes, it's about recognizing that reality.

Unless you're going to somehow _prevent_ the
use of plists with keys that are other than,
e.g., symbols - in particular keys that are
strings, adding a TEST arg makes sense, IMO.