Add a new test application for the FSFS private API.
Implement a test for svn_fs_fs__get_stats.

* build.conf
  (fs-fs-private-test): Declare new build target for the test.
  (__ALL_TESTS__): Add it to the other binary tests.

* subversion/tests/libsvn_fs_fs/fs-fs-private-test.c
  (): New test application.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632796 13f79535-47bb-0310-9956-ffa450edef68