Generate result filenames that are easier to bulk update

Currently, this plugin organizes its outputs as given in this example:

results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/baseline.png
results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/result-failed-diff.png
results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/result.png

https://github.com/matplotlib/pytest-mpl/blob/2611bd020b523ab4fff1c9fa7db936b68dad7113/pytest_mpl/plugin.py#L425

But what result.png compares to is named as such:

https://github.com/astropy/astropy-data/blob/gh-pages/testing/astropy/2019-08-02T11:38:58.288466/3.1.x/test_tick_angles_non_square_axes.png

As a result, to save the new result.png as a new "truth," I have to manually rename the file. When I have to bulk update like 50 of these, there is a high risk of human error of renaming the wrong stuff.

Would be more dev friendly to perhaps re-organize the naming to be something like this?

results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/baseline.png
results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/result-failed-diff.png
results/astropy.visualization.wcsaxes.tests.test_images.test_tick_angles_non_square_axes/test_tick_angles_non_square_axes.png

Inizia in pytest_mpl/plugin.py intorno alla riga 425, dove vengono organizzati i nomi dei file dei risultati. Confronta il nome attuale result.png con il nome del file specifico per il test richiesto, quindi verifica che gli output generati utilizzino il nome del test, così gli aggiornamenti massivi di truth non richiederanno di rinominare manualmente i file.