[[[
Add a test of svn_subst_translate_string2() to the subst_translate test suite.

As discussed at:
  http://thread.gmane.org/gmane.comp.version-control.subversion.devel/125782
    Message-ID: <AANLkTimo=NpF_FqB+PBPkeifXjbnsORGYkZ5npCAxyVz@mail.gmail.com>

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_svn_subst_translate_string2_null_encoding_helper): New function. It is
    the core of the new svn_subst_translate_string2_null_encoding test.
  (test_svn_subst_translate_string2_null_encoding): New test that tests
    svn_subst_translate_string2() with ENCODING set to NULL.
  (test_funcs): Add test_svn_subst_translate_string2_null_encoding().
]]]