Follow-up to r1606840: Update another svnadmin verify check.

* subversion/tests/cmdline/svnadmin_tests.py
  (verify_keep_going): Expect separate output for repos with low-level
                       checksum checks vs. those without. 

* subversion/tests/cmdline/svntest/verify.py
  (RegexListOutput.extend): Add trivial new method.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606902 13f79535-47bb-0310-9956-ffa450edef68