Add an XFail test for svn:external switching repositories.
Add an XFail test for svn:external switching repositories.

* subversion/tests/cmdline/externals_tests.py
  (relegate_external): New.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion