LU-17679 tests: make stdout unbuffered in sanity/851

LU-17679 tests: make stdout unbuffered in sanity/851

Make stdout unbuffered in sanity/test_851.
So that the output can be detected ASAP.

Fixes: 7101742b459 ("LU-17490 tests: verify fanotify works for lustre")
Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Test-Parameters: testlist=sanity env=ONLY=851,ONLY_REPEAT=1000
Change-Id: I852a73b6bbdcbae7dfe1704183d4cb3c8200be43
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55904
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>