Generate result filenames that are easier to bulk update

Nobody has claimed this yet.

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
```

Start in pytest_mpl/plugin.py around line 425, where the result filenames are organized. Compare the current result.png naming with the requested test-specific filename, then verify that generated outputs use the test name so bulk truth updates do not require manual renaming.

Written by the indexing model from the issue text.