Partly revert "remove now-obsolete use of skip_doctest outside core"

This partly reverts commit 02234da0d461f5ddee142d51952dc10a86b8074e.

test_decorators.py is intended to test skip_doctest, the removing was wrong.

IPython/testing/tests/test_decorators.py

@@ -11,6 +11,7 b' import nose.tools as nt'
 
 # Our own
 from IPython.testing import decorators as dec
+from IPython.testing.skipdoctest import skip_doctest
 
 #-----------------------------------------------------------------------------
 # Utilities
@@ -59,6 +60,7 b' def test_deliberately_broken2():'
 
 # Verify that we can correctly skip the doctest for a function at will, but
 # that the docstring itself is NOT destroyed by the decorator.
+@skip_doctest
 def doctest_bad(x,y=1,**k):
     """A function whose doctest we need to skip.
 
@@ -106,6 +108,7 b' class FooClass(object):'
     2
     """
 
+    @skip_doctest
     def __init__(self,x):
         """Make a FooClass.
 
@@ -117,6 +120,7 b' class FooClass(object):'
         print('Making a FooClass.')
         self.x = x
         
+    @skip_doctest
     def bar(self,y):
         """Example: