Exchange tests

[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 OpenLand.

Briefly, testing the str() output of a ggplot-based object is brittle, as it might change as ggplot2's internals change.
This PR changes these tests by a simple test of whether the output is 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 February. The progress of the release can be tracked in tidyverse/ggplot2#5588. We hope that this PR might help OpenLand get out a fix if necessary.

[teunbrand]

This is just a friendly reminder that the release of ggplot2 is scheduled soon.