Dump failing images to a directory about pytest-pyvista

[MatthewFlamm]

Another option is to save all image files during a test to a folder (default to a temporary folder). Then you could upload this to an artifact in CI. This would lead to larger artifacts, but also has other advantages, for example being able to inspect for false negatives. I don't have a feeling one way or the other.

[MatthewFlamm]

To expand upon this: I think we should have two configurable directories. 1) the stored image cache, 2) the generated images for a single run.

1. stored image cache. Can there be a universal way to set this? It should be related to the testing code, which could be hierarchical, so tricky to access here. If it is required to be explicitly set by the user, we should check that this is done, or raise an error.
2. generated images. This could be in the shared appdirs by default.

It would also be great in the future to allow for an optional check that all the files in the cache dir have a corresponding file in generated dir, and vice versa. This would ensure that everything is working as expected during testing.

[banesullivan]

I am doing this in pyvista/pyvista#3609 which would need to migrate here

[banesullivan]

Good point. I'd rather this be to save all produced images, not just failing ones