Add support for conditional XFail wrappers.
Add support for conditional XFail wrappers.

* subversion/tests/svn_test.h
  (global): add new macro SVN_TEST_XFAIL_COND.

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