Provide a simple test for the new r/o mode in config_t.
This requires the addition of private API to access internal state.

* subversion/include/private/svn_subr_private.h
  (svn_config__is_read_only,
   svn_config__is_expanded): declare new private API

* subversion/libsvn_subr/config.c
  (svn_config__is_expanded): implement new API

* subversion/libsvn_subr/config_file.c
  (svn_config__is_read_only): implement new API

* subversion/tests/libsvn_subr/config-test.c
  (test_read_only_mode): new test case
  (test_funcs): register it

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1535721 13f79535-47bb-0310-9956-ffa450edef68