LU-13314 utils: fix lfs find time calculation margin

LU-13314 utils: fix lfs find time calculation margin

Allow a larger margin when checking files that are years old.

Re-enable sanity test_56ob.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Id74e4b737ebb3a2b721d3e3b6d79dffe703ebbe5
Reviewed-on: https://review.whamcloud.com/39433
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>