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