Ensure backward-compatibility for coverage option

@mdboom - affiliated packages using the latest dev version of astropy will now get:

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "photutils/__init__.py", line 109, in test
    coverage=coverage, open_files=open_files, **kwargs)
TypeError: run_tests() got an unexpected keyword argument 'coverage'
```

can we make the option backward-compatible?