Compatibility with ggplot2 3.6.0

[teunbrand]

Description

Hi there,

Apologies for not posting an issue first and for noncompliance with your checklist.
The ggplot2 package is planning an update for around May 2025 and a reverse dependency test identified a problem with the migraph 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"))

Best,
Teun

Checklist:

• Documentation
  • Any longer functions are commented inline so that it is easier to debug in the future
  • Any new or modified functions or data have roxygen style documentation in their .R scripts
  • DESCRIPTION file version is bumped by the appropriate increment (major, minor, patch)
• PR form
  • Title indicates expected version number
  • PR description above and the NEWS.md file are aligned
  • Any closed, fixed, or related issues are referenced and described, e.g. "Fixed #0 by adding A"
• PR checks all pass for latest commit
  • Package builds on my OS without issues
  • CodeFactor check: Package improves or maintains good style
  • Package builds on Mac
  • Package builds on Windows
  • Package builds on Linux
  • CodeCov check: Package improves or maintains good test coverage