cls_lock: fix duration test

It's possible for us to just be really slow when getting the reply to the
first op or doing the second op, resulting in a successful lock.  If we
do get a success, assert that at least that amount of time has passed to
avoid any false positives.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>