Address PendingDeprecationWarnings in watershed tests

Resolves the following warnings raised from the watershed algorithm test suite:

```
./scikit-image/skimage/morphology/tests/test_watershed.py:157: PendingDeprecationWarning: Please use assertTrue instead.
  self.failUnless(error < eps)
./scikit-image/skimage/morphology/tests/test_watershed.py:192: PendingDeprecationWarning: Please use assertTrue instead.
  self.failUnless(error < eps)
./scikit-image/skimage/morphology/tests/test_watershed.py:227: PendingDeprecationWarning: Please use assertTrue instead.
  self.failUnless(error < eps)
./scikit-image/skimage/morphology/tests/test_watershed.py:262: PendingDeprecationWarning: Please use assertTrue instead.
  self.failUnless(error < eps)
./scikit-image/skimage/morphology/tests/test_watershed.py:294: PendingDeprecationWarning: Please use assertTrue instead.
  self.failUnless(error < eps)
```