Make svnadmin-tests pass for 1.5 to 1.9 compat runs.

* subversion/tests/cmdline/svntest/main.py
  (fs_has_rep_sharing,
   fs_has_pack): New feature presence checks that we can use
                 e.g. for @SkipUnless conditions.

* subversion/tests/cmdline/svntest/verify.py
  (RegexOutput.insert,
   RegexListOutput.insert): Add these methods that allow us to e.g.
                            prepend lines to svadmin verify outputs
                            depending on metadata checks being done.
  (RegexListOutput.__init__): Allow us to start with an empty list.

* subversion/tests/cmdline/svnadmin_tests.py
  (verify_windows_paths_in_repos): Use the new feature checks instead
                                   of local logic.
  (load_ranges): Filter sha1 checksums from the expected dump files
                 when we use pre-1.5 repositories.
  (hotcopy_incremental_packed): Skip this when the repo doesn't support
                                packing.
  (verify_keep_going,
   verify_invalid_path_changes,
   verify_denormalized_names): Insert "verifying metadata" patterns
                               into expected output depending on repo
                               capabilities.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547484 13f79535-47bb-0310-9956-ffa450edef68