setup.py test -t option not working as expected for docs

If I do:

```
$ python setup.py test -t docs/table/operations.rst
```

I get 

```
ERROR: file not found: /Users/tom/Dropbox/Code/development/Astropy/astropy/docs/docs/table/operations.rst
```

It seems to be adding ``docs`` to the path. This is probably trivial, but I will not have time to look until next week.

cc @mdboom