FIX TEST test_forward: change subjects_dir

As suggested in https://github.com/mne-tools/mne-python/pull/434#issuecomment-13411325

I still get two errors in the tests on master:

```Python

ERROR: Test reading labels from parc. by comparing with mne_annot2labels
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/7.3/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Library/Frameworks/Python.framework/Versions/7.3/lib/python2.7/site-packages/numpy/testing/decorators.py", line 146, in skipper_func
    return f(*args, **kwargs)
  File "/Users/christian/Documents/Eclipse/projects/mne-python/mne/tests/test_label.py", line 183, in test_labels_from_parc_annot2labels
    labels_mne = _mne_annot2labels('sample', subjects_dir, 'aparc')
  File "/Users/christian/Documents/Eclipse/projects/mne-python/mne/tests/test_label.py", line 172, in _mne_annot2labels
    % st)
RuntimeError: mne_annot2labels non-zero exit status 256
```
and
```Python
ERROR: Test sensitivity map computation
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/7.3/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/christian/Documents/Eclipse/projects/mne-python/mne/tests/test_proj.py", line 42, in test_sensitivity_maps
    w_lh = mne.read_w(sensmap_fname % (ch_type, 'lh'))
  File "/Users/christian/Documents/Eclipse/projects/mne-python/mne/source_estimate.py", line 155, in read_w
    fid = open(filename, 'rb')
IOError: [Errno 2] No such file or directory: u'/Users/christian/Documents/Eclipse/projects/mne-python/examples/MNE-sample-data/MEG/sample/sample_audvis-eeg-oct-6-fwd-sensmap-lh.w'
```