Compatibility with ggplot2 3.6.0

[teunbrand]

Hi there,

Apologies for not posting an issue first.
The ggplot2 package is planning an update for around May 2025 and a reverse dependency test identified a problem with the BayesMallows package.
The details are explained in tidyverse/ggplot2#6290, but essentially ggplot2 doesn't populate the plot$labels field before plot building anymore, which violates some test assumptions in this package.

This PR changes the tests to be compatible with both versions of ggplot2.
You can test the changes yourself with the development version of ggplot2 (pak::pak("tidyverse/ggplot2"))
While preparing this PR, I found additional errors that appeared unrelated to ggplot2, but I'm not sure if they're of any concern.

Best,
Teun

[osorensen]

Thanks a lot for making the necessary changes for me!

[teunbrand]

Whoops there was a small mistake on my end. It is fixed now.

Thanks a lot for making the necessary changes for me!

Well we're just hoping that it makes it easier to push a new release to CRAN should that be necessary :)