enable IPython formatter when testing it

IPython/core/tests/test_formatters.py

@@ -407,6 +407,9 b' def test_ipython_display_formatter():'
         def _ipython_display_(self):
             raise NotImplementedError
     
+    save_enabled = f.ipython_display_formatter.enabled
+    f.ipython_display_formatter.enabled = True
+    
     yes = SelfDisplaying()
     no = NotSelfDisplaying()
     
@@ -420,6 +423,9 b' def test_ipython_display_formatter():'
     nt.assert_equal(md, {})
     nt.assert_equal(catcher, [yes])
 
+    f.ipython_display_formatter.enabled = save_enabled
+
+
 def test_json_as_string_deprecated():
     class JSONString(object):
         def _repr_json_(self):