python setup.py test -n option is broken

`python setup.py test --help` claims that one can use the `-n` option instead of `--parallel`:
```
$ python setup.py test --help
...
  --parallel (-n)         Run the tests in parallel on the specified number of
                          CPUs.  If negative, all the cores on the machine
                          will be used.  Requires the pytest-xdist plugin.
...
```

But actually this doesn't work with current astropy master:
```
$ python setup.py test -n
...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "astropy/__init__.py", line 196, in test
    skip_docs=skip_docs)
  File "astropy/tests/helper.py", line 231, in run_tests
    "parallel must be an int, got {0}".format(parallel))
ValueError: parallel must be an int, got 
```
```
$ python setup.py test -n 4
invalid command name '4'
```
```
$ python setup.py test -n4
error: option -4 not recognized
```
Am I using it incorrectly or is this broken.
(I guess the `-n` shortcut could simply be removed?)