unit test LFNIndex::remove_object and LFNIndex::lfn_unlink

When the object name is short, check that the corresponding file is
::unlink()ed. When the object name is long, there may be multiple files
with the same name, modulo the anti-collision number showing just before
the FILENAME_COOKIE. The following scenarii are tested:

 * there only is one file

 * there are multiple files and the last one is removed

 * there are multiple files and the last one is moved in place of the
   file that is to be removed

lfn_unlink and remove_object are tested together because
lfn_unlink is a private function and remove_object is a protected function
that does very little beside calling lfn_unlink

http://tracker.ceph.com/issues/4560 refs #4560

Signed-off-by: Loic Dachary <loic@dachary.org>