test: Fix tiering test cases to use ---force-nonempty

Fixes: #7608

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge pull request #1376 from ceph/wip-7608

test: Fix tiering test cases to use ---force-nonempty

Reviewed-by: Sage Weil <sage@inktank.com>