Fix svnadmin_tests.py with Python versions before 2.6.

* subversion/tests/cmdline/svnadmin_tests.py
  (modify_time): Use old style formatting rather than the format() string
    method added in 2.6.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606770 13f79535-47bb-0310-9956-ffa450edef68