Add tests for values, limits, and breaks in manual scale

[banfai]

Issue introduced in #4471, if there are extra named values in a manual scale then they will show up on the legend even with drop = TRUE.

Question: should these values be forced with drop = FALSE? (currently it's possible)

[msberends]

Question: should these values be forced with drop = FALSE? (currently it's possible)

I would very much like so. Probably a nice option to add legend items not present in the data if the plot is part of a series, or e.g. to make it obvious that a factor contains levels that have zero observations.

[yutannihilation]

@banfai
Hi, thank you for creating this pull request. Recently, we got an alternative pull request (#4619) that tackles the same issue. We are getting to feel #4619 is preferred in that the change is only on manual scale, while your pull request modifies the whole Scale.

Would you mind tweaking this pull request to contain only the tests? These are helpful to check if #4619 does the right thing, and, what's more important, I'd like to make it clear that you are a contributor by merging this. The CI checks will of course fail, but it's fine. If you don't have time, I can do it.

[banfai]

Hi @yutannihilation @teunbrand

Thanks for having a look at this. I have seen @teunbrand 's comment earlier on 4511 and agreed that it was a leaner solution, but haven't had time to look deeper or change this PR.
I have removed the changes in this and added some more tests. These pass on my computer with #4619 included.

[yutannihilation]

Thank you!