Parallel testing fixes

Parallel testing doesn't work on my Mac at the moment because apparently there's two problems with non-deterministic test case collection:
https://gist.github.com/cdeil/9611d070beb593ce0f4c

967ea8c fixes the first issue, but there's still this one in `astropy.table`:
```
INTERNALERROR> -astropy/table/table.py::astropy.table.table.Row
INTERNALERROR> +astropy/table/table.py::astropy.table.table.Table.Row
```

@taldcroft Can you reproduce? Any idea how to fix this?
(I think #1402 is unrelated?)