Support hierarchical tests

Hi,

I am working a lot with images and therefore love the idea of the project. I played around with it a little bit and noticed something.

When I group my tests in classes, the image file gets named only after the function that calls it.

Example:

class Test_Imageutils:
    @pytest.mark.mpl_image_compare
    def test_put_text(self):
        from skimage import data
        astronaut = data.astronaut()
        annotated = imageutils.put_text(astronaut.copy(), "hi", loc="center")
        fig, ax = plt.subplots(1, 1, figsize=(6, 6))
        ax.imshow(annotated)
        return fig

And the image in baseline is named test_put_text.png

I think it would be a great idea to add an option to write the full hierarchy, either into folders or into the file name. For example

• baseline/Test_Imageutils/test_put_text.png
• baseline/Test_Imageutils-test_put_text.png

What do you think?

I could dig a little into the code and come up with a PR if anyone thinks it is a good idea.

Start by locating the code that derives baseline image names from pytest tests, using the Test_Imageutils/test_put_text example as the expected case. Compare the folder and filename approaches described in the issue, then define the option and verify that hierarchical names are generated consistently.