Fix the subversion version check in FSX tests.

* subversion/tests/libsvn_fs_x/fs-x-pack-test.c
  (file_hint_at_shard_boundary,
   test_info,
   test_reps): FSX is available in 1.9+ but not 1.8-

Found by: danielsh

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1513012 13f79535-47bb-0310-9956-ffa450edef68