Extend XFail test for issue #4085, "external can shadow a versioned directory"

* subversion/tests/cmdline/externals_tests.py
  (shadowing): Try to remove the shadowed directory and update the working copy.
   This shows that the working copy remains wedged even if the user attempts
   to recover from the situation by removing the offending directory.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628431 13f79535-47bb-0310-9956-ffa450edef68