Provide a test case for the new svn_fs_fs__load_index API.

* subversion/tests/libsvn_fs_fs/fs-fs-private-test.c
  (receive_index): New receiver utility to read the old index contents.
  (load_index): The new test case.
  (test_funcs): Register the new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632870 13f79535-47bb-0310-9956-ffa450edef68