Provide a comprehensive test for the config_pool API.

* subversion/tests/libsvn_repos
  svn:ignore temporary config files

* subversion/tests/libsvn_repos/repos-test.c
  (test_config_pool): new test
  (test_funcs): register it

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537358 13f79535-47bb-0310-9956-ffa450edef68