Unify svnadmin verify --keep-going check for all FSFS formats.

* subversion/tests/cmdline/svnadmin_tests.py
  (verify_keep_going): Corrupt at the beginning of the rep file
                       instead of appending to it.  We now get
                       the same error messages for all formats.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604415 13f79535-47bb-0310-9956-ffa450edef68