Teach our cmdline tests to use a pre-cooked FSFSv6 repository
created by a 1.8 svnadmin.

* subversion/tests/cmdline/svntest/main.py
  (create_repos): If the FSFS version option is set, find and use an
   appropriate empty template repository instead of creating one
   using the current svnadmin binary.
  (fs_has_unique_freeze): New predicate.

* subversion/tests/cmdline/svnadmin_tests.py
  (freeze_same_uuid): Use the new predicate; the test now covers BDB as well.

* subversion/tests/templates/empty-fsfs-v6.zip: New;
   zipped empty repository created by svnadmin 1.8.10 on a Mac.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619399 13f79535-47bb-0310-9956-ffa450edef68