Follow up on r1567819, use absolute paths so that autoprop_tests.py 28 passes

* subversion/tests/cmdline/autoprop_tests.py
  (autoprops_test, inheritable_autoprops_test,
   svn_prop_inheritable_autoprops_add_versioned_target): Use absolute paths for
    the tmp_dir so that config_dir can still be found even if we have changed
    working directories as we do in test 28.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567848 13f79535-47bb-0310-9956-ffa450edef68