Fix test so it passes even when the previous run wasn't cleaned up.
Fix test so it passes even when the previous run wasn't cleaned up.

Found by: rooneg

* subversion/tests/cmdline/authz_tests.py
  (authz_partial_export_test): cleanup remains of a previous 
   test run.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion