LU-15365 tests: conf-sanity/115 to cleanup properly

LU-15365 tests: conf-sanity/115 to cleanup properly

when /tmp can't fit large MDT filesystem the image
should be removed after all.

Test-Parameters: trivial testlist=conf-sanity env=ONLY=115
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ifb0bd201156f4beb665f3c38aa02d44802b13bbf
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/45836
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>