Update the testcase to handle --quiet also.

* subversion/tests/cmdline/svnadmin_tests.py
  (verify_keep_going): Check the behavior of --quiet option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/verify-keep-going@1466809 13f79535-47bb-0310-9956-ffa450edef68