Fix a long-standing pool handling bug when expanding config values.

* subversion/libsvn_subr/config.c
  (make_string_from_option): Copy expanded value whenever the pool
   is not the expanded values pool.

* subversion/tests/libsvn_subr/config-test.c
  (test_expand): Tweak comment.
  (test_funcs): Mark test_expand PASS.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1646797@1646799 13f79535-47bb-0310-9956-ffa450edef68