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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

* 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. It wraps
    test_svn_subst_translate_string2_null_encoding_helper() to ensure that the
    system-default character encoding is set to either CP-1252 or ISO-8859-1
    before test_svn_subst_translate_string2_null_encoding_helper() is called,
    later restoring the original system-default character encoding.
  (test_funcs): Add test_svn_subst_translate_string2_null_encoding().
]]]