Avoid occasional spurious FAILs when running autoprop_tests in parallel.

* subversion/tests/cmdline/autoprop_tests.py
  (autoprops_test): Use subdir of local_tmp.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567752 13f79535-47bb-0310-9956-ffa450edef68