Construct paths in a proper way in the config tests.

* subversion/tests/libsvn_subr/config-test.c
  (includes): Add svn_dirent_uri.h.
  (test_text_retrieval,
   test_boolean_retrieval,
   test_has_section_case_insensitive,
   test_has_section_case_sensitive,
   test_has_option_case_sensitive,
   test_stream_interface,
   test_read_only_mode): Use svn_dirent_join to join paths.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537225 13f79535-47bb-0310-9956-ffa450edef68