Add an XFAIL regression test for issue 4415, XML-unsafe svn:author.

* subversion/tests/cmdline/prop_tests.py
  (xml_unsafe_author): New.
  (test_list): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515784 13f79535-47bb-0310-9956-ffa450edef68