Testing the `plot$labels` field

[teunbrand]

This problem was identified in #6287 and affects 32 packages.

Many packages use the labels plot field in their tests to see if their code has correctly constructed their plot.
We now only populate this field with the labs() function or when the plot is build, and no longer when layers are added automatically.
We have an accessor in place that people can use (#6078).

Should we start sending out PRs to affected packages?

[teunbrand]

TODO: Send out PRs to packages

[teunbrand]

A suggestion was raised in tidyverts/feasts#166 (comment).
Essentially, annotate() populates labels and it would be nice if these were deprioritised in favour of regular layers.