Add a FS API level test for svn_fs_paths_changed2().

The other part of the 'svn log' functionality is the history
traversal, which is already covered by log_tests.py.

* subversion/tests/libsvn_fs/fs-test.c
  (verify_root_prop_change): New verification utility.
  (test_paths_changed): The new test.
  (test_funcs): Register new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631047 13f79535-47bb-0310-9956-ffa450edef68