Make the svnadmin tests succed with FS_TYPE=fsfs-v6.

* subversion/tests/cmdline/svntest/main.py
  (tests_use_prepacakaged_repository): New predicate.
* subversion/tests/cmdline/svnadmin_tests.py
  (create_in_repo_subdir): Skip if the new predicate applies.
  (load_ignore_dates): Ignore r0, its properties aren't affected by the
   'svnadmin load' anyway.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619403 13f79535-47bb-0310-9956-ffa450edef68