BUG astropy.test() does not run docstring tests

Yet another follow-up on #1752 and #1789. Following a suggestion of @mdboom, it seems that currently if one runs `astropy.test()`, all docstring tests are skipped, unlike what happens if one does `python setup.py test`.

One can see that from looking at the output, where the latter includes lines like,
```
astropy/units/core.py ....
```
when the former does not -- though it does have a strange ending with `WARNING`
repeated multiple times:
```
.
.
.
============ 4807 passed, 88 skipped, 13 xfailed in 139.48 seconds =============
WARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNINGWARNING0
```