Tests expectation with ggplot2 3.5.0

[teunbrand]

Hello there,

We have been preparing a new release of ggplot2 and during a reverse dependency check, it became apparent that the prospective ggplot2 3.5.0 would break assignPOP.

This PR updates a few tests in assignPOP to deal with the latest changes. In summary, the str(plot) object can change when ggplot2 changes its internals, making these tests fragile. These tests have been replaced to expect a ggplot object.

To test the code changes with the release candidate, you can install it with the code below:

remotes::install_github("tidyverse/ggplot2", ref = remotes::github_pull("5592"))

The release of ggplot2 3.5.0 is scheduled for the 12th of Februari. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help assignPOP get out a fix if necessary.

[teunbrand]

This is just a kind reminder that we plan to release ggplot 3.5.0 soon.