Skip notebook 'static' dir in test suite.

There's nothing there we should be running python tests on, and
occasionally python files may appear there that cause problems to nose
(e.g. mathjax ships a non-py3 valid conf.py that breaks py3 testing).

IPython/testing/iptest.py

@@ -232,6 +232,11 b' def make_exclude():'
                   ipjoin('lib', 'inputhook'),
                   # Config files aren't really importable stand-alone
                   ipjoin('config', 'profile'),
+                  # The notebook 'static' directory contains JS, css and other
+                  # files for web serving.  Occasionally projects may put a .py
+                  # file in there (MathJax ships a conf.py), so we might as
+                  # well play it safe and skip the whole thing.
+                  ipjoin('frontend', 'html', 'notebook', 'static')
                   ]
     if not have['sqlite3']:
         exclusions.append(ipjoin('core', 'tests', 'test_history'))