Instead of manually fiddling with FSFS index data in our test suite,
use the svnfsfs tool to read and rewrite the indexes after we manipulated
the revision contents.

* subversion/tests/cmdline/svnadmin_tests.py
  (FSFS_Index): New utility class handling all the index access.
  (read_l2p): Drop obsolete utility.
  (set_changed_path_list): Use the new class to read the index data and
                           to keep it consistent with our manipulated
                           rev content.
  (verify_invalid_path_changes): Update expected errors as we don't get
                                 low-level checksum mismatches anymore.

* subversion/tests/cmdline/svntest/main.py
  (svnfsfs_binary): New tool constant similar to svnauthz-binary.
  (run_svnfsfs): New runner utility for the new binary.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606743 13f79535-47bb-0310-9956-ffa450edef68