Pass the correct baton to the open-tunnel callback.

* subversion/libsvn_ra_svn/client.c (open_session):
   Use callbakcs->tunnel_baton, not callbacks_baton, in the open-tunnel call.
* subversion/tests/libsvn_ra/ra-test.c: Add checks to make sure that the
   tunnel baton and context get propagated through the callbacks.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532126 13f79535-47bb-0310-9956-ffa450edef68