Add a regression test for issue #4411, "parent-relative file external
can corrupt working copy".

* subversion/tests/cmdline/externals_tests.py
  (switch_parent_relative_file_external, test_list): New test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1513479 13f79535-47bb-0310-9956-ffa450edef68