Test that 'svnadmin freeze' is nestable.

Review by: julianfoad

* subversion/tests/cmdline/svnadmin_tests.py
  (freeze_freeze): New test.
  (test_list): Run it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1585535 13f79535-47bb-0310-9956-ffa450edef68