test/librbd: make diff-iterate clone tests exercise fast-diff mode

test/librbd: make diff-iterate clone tests exercise fast-diff mode

The fast-diff feature wasn't propagated to the clone so these tests
were exercising the slow list_snaps path no matter what RBD_FEATURES
value was supplied to ceph_test_librbd.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit ceb13d76f2b3aba7209e85f3354970c072997742)

Conflicts:
	src/test/librbd/test_librbd.cc [ commit d1c82d55827e ("librbd:
	  enable image cache after getting exclusive lock") not in
	  octopus ]