make fix for tab_comnpletion test

due to an error in understanding of dynamic trait
instantiation:
output spec is actually instantiated in
IOBase._outputs(), which calls Function._add_output_traits,
which only will add things in self._output_names

Author: leej3

nipype/interfaces/base/tests/test_specs.py

@@ -106,7 +106,7 @@ def extract_func(list_out):
     assert(sorted(list_extract.outputs.__all__) == expected_output)
 
     # Add trait and retest
-    list_extract.outputs.add_trait("added_out_trait","val")
+    list_extract._interface._output_names.append('added_out_trait')
     expected_output = sorted(['added_out_trait',*expected_output])
     assert(sorted(list_extract.outputs.__all__) == expected_output)