Fix tests for #1257

The test of blob_doh() set an unrealistically strict threshold for the
accuracy of the estimated blob radius, causing the test to fail with the
true floating-point value (previously, the test passed only because
sigma was truncated to integer). Increasing the threshold fixes the
problem.

skimage/feature/tests/test_blob.py

@@ -141,7 +141,7 @@ def test_blob_doh():
 
     radius = lambda x: x[2]
     s = sorted(blobs, key=radius)
-    thresh = 3
+    thresh = 4
 
     b = s[0]
     assert abs(b[0] - 400) <= thresh