LU-17721 tests: improve sanity-flr/210b

LU-17721 tests: improve sanity-flr/210b

do not use total space counters - it's proven to be an unreliable
approach. instead check whether specific object is really removed
(using debugfs).

Test-Parameters: trivial env=ONLY=210b,ONLY_REPEAT=20 testlist=sanity-flr
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I8d7af4ef5404115ce94040960b3cae9c05c9832f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54719
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>