Generate result filenames that are easier to bulk update

Dieses Issue hat noch niemand übernommen.

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
```

Beginne in pytest_mpl/plugin.py ungefähr bei Zeile 425, wo die Dateinamen der Ergebnisse organisiert werden. Vergleiche die aktuelle Benennung result.png mit dem angeforderten testspezifischen Dateinamen und überprüfe anschließend, dass generierte Ausgaben den Testnamen verwenden, damit gebündelte Truth-Updates kein manuelles Umbenennen erfordern.

Vom Indexierungsmodell aus dem Issue-Text verfasst.