Use a unique UUID for each repository used by the test suite.

* subversion/tests/cmdline/svntest/actions.py
  (guarantee_greek_repository): Set a fresh UUID on the copied repository.
   Before this change, every repository used by the test suite shared the
   same UUID. Which has bad effects when trying to run the test suite with
   memcached support enabled since the UUID is part of the cache key. 


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1585678 13f79535-47bb-0310-9956-ffa450edef68