Add a FS test that covers svn_fs_[props|content]_[changed|different].
Use an MD5 collision to cover as much code as we can.

* subversion/tests/libsvn_fs/fs-test.c
  (compare_contents): New test.
  (test_funcs): Register the new test.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1636477 13f79535-47bb-0310-9956-ffa450edef68