test/libcephfs: ensure that we set the mode of the libcephfs root before testing

test/libcephfs: ensure that we set the mode of the libcephfs root before testing

Patrick noticed that the libcephfs tests failed to run unless they were
run after the ceph-fuse tests had run. The ceph-fuse tests set the root
of the share to mode 01777, but the libcephfs tests don't do that even
though they rely on the directory being world-writable.

Fix this by adding a new main() function for ceph_test_libcephfs, and
calling a function (copied from access.cc) that resets the permissions
of the root before running the tests.

Fixes: http://tracker.ceph.com/issues/18013
Signed-off-by: Jeff Layton <jlayton@redhat.com>