code for generating and comparing images

Here ``_ASTROPY_PACKAGE_NAME_`` is ``wcsaxes`` (confusing) so this is equivalent to:

```
        'wcsaxes.tests': ['coveragerc'],
        'wcsaxes.tests': ['baseline_images/*.png']}
```

hence the first entry for ``coveragerc`` is being ignored, so the ``coveragerc`` file isn't being copied over for the tests. Instead, try using:

```
_ASTROPY_PACKAGE_NAME_ + '.tests': ['coveragerc', 'baseline_images/*.png'],
```