Add test-level option to choose image name for comparison

[MatthewFlamm]

This would enable reducing duplicate images in tests. For example, tests in pyvista/pyvista#5428 generate multiple images that are duplicates of one another. It could be something like:

def test_identical_images(verify_image_cache):
    verify_image_cache.file_compare = "identical_images"
    # generate image with first plot and compare to "identical_images.png"
    verify_image_cache.file_compare = "identical_images"
    # generate image with second plot and compare to same "identical_images.png"