Add a new test that demonstrates a bug in the config expansion code.

* subversion/tests/libsvn_subr/config-test.c
  (test_expand): New test.
  (test_funcs): Add new test, mark WIMP.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1646785 13f79535-47bb-0310-9956-ffa450edef68