Replace a doc test with some regular unit tests.

skimage/filters/_gaussian.py

@@ -74,10 +74,6 @@ def gaussian_filter(image, sigma, output=None, mode='nearest', cval=0,
     array([[ 0.05855018,  0.09653293,  0.05855018],
            [ 0.09653293,  0.15915589,  0.09653293],
            [ 0.05855018,  0.09653293,  0.05855018]])
-    >>> gaussian_filter(a, sigma=-1)
-    Traceback (most recent call last):
-    ...
-    ValueError: Sigma values less than zero are not valid
     >>> # Several modes are possible for handling boundaries
     >>> gaussian_filter(a, sigma=1, mode='reflect')
     array([[ 0.08767308,  0.12075024,  0.08767308],

skimage/filters/tests/test_gaussian.py

@@ -1,8 +1,23 @@
+from nose.tools import raises
 import numpy as np
 from skimage.filters._gaussian import gaussian_filter
 from skimage._shared._warnings import expected_warnings
 
 
+@raises(ValueError)
+def test_negative_sigma():
+    a = np.zeros((3, 3))
+    a[1, 1] = 1.
+    gaussian_filter(a, -1.0)
+
+
+@raises(ValueError)
+def test_multiple_negative_sigma():
+    a = np.zeros((3, 3))
+    a[1, 1] = 1.
+    gaussian_filter(a, [-1.0, 1.0])
+
+
 def test_null_sigma():
     a = np.zeros((3, 3))
     a[1, 1] = 1.