LU-18758 tests: test lfs migrate --block on changing file

LU-18758 tests: test lfs migrate --block on changing file

Run 'lfs migrate --block' on a continously changing file
to check wether '--block' option is broken.

Test-Parameters: trivial env="ONLY=56xab,ONLY_REPEAT=500"
Signed-off-by: Lei Feng <flei@whamcloud.com>
Change-Id: I48492b6296f81d0cbe3bc83fad2ffeb855dff1aa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58299
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>