bug#63627: Improve plstore.el and fix various issues of it

[Jens Schmidt via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On 2023-09-07  11:12, Eli Zaretskii wrote:
>> Date: Mon, 4 Sep 2023 00:22:52 +0200
>> Cc: 63627@debbugs.gnu.org
>> From: Jens Schmidt <jschmidt4gnu@vodafonemail.de>
> 
> If the changeset is now ready to be installed on master, please tell.

I consider it ready to be installed on master, yes, and self-contained,
but:

>> The rest of the tests should be more or less standard.  Except
>> probably for the fact that there is still much infrastructure and few
>> actual tests, but I plan to change that.
> 
> This seems to imply that you are still working on this changeset?

I plan for plstore.el:

1. Two bug fixes on emacs-29 that I would like to add tests for on
   master once the bug fixes have made their way to master.

2. More tests and new features on emacs-master.

All of the above will still take some time to implement due to time
constraints.

For all of the above it would be convenient to have the infrastructure
of the plstore tests committed on master or at least agreed upon so
that I can build on that.

Not sure how you prefer to get such long-running projects presented -
as a series of minor (but self-contained!) patches over time or at
once as one big set of patches.

Thanks.