Modify reopen_modify so it demonstrates the FSFS txn_dir_cache
out-of-date problem.

* subversion/tests/libsvn_fs/fs-test.c
  (struct reopen_modify_baton_t): New.
  (reopen_modify_child): New.
  (reopen_modify): Invoke a thread to make second change.
  (test_funcs): Mark reopen_modify WIMP.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1617500 13f79535-47bb-0310-9956-ffa450edef68