Add XFAIL regression test for keyword expansion with long keywords.

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_one_long_keyword): New helper.
  (test_svn_subst_long_keywords): New test.
  (test_funcs): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467221 13f79535-47bb-0310-9956-ffa450edef68