Let the FS test repo tree validation code verify that node
types are reported consistently.  Cover svn_fs_check_path,
svn_fs_is_file and svn_fs_file_length with that test.

* subversion/tests/svn_test_fs.c
  (validate_tree_entry): Check that all 3 node classification
                         functions return consistent results.
                         Check that reported file length is
                         consistent with file contents.
  (svn_test__validate_tree): Check that unavailable nodes are
                             reported correctly and consistenly.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1633965 13f79535-47bb-0310-9956-ffa450edef68