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

[Philipp]

> So does anybody have an opinion here?  I think I'm in favour of adding a
> comparison function for all three `plist-*' functions.

I'm not a fan of this since it would mean that these functions will no longer 
be side-effect-free.
Adding a function argument to a function means that the function can no longer 
really promise anything, so it leads to code that's harder to reason about.