LU-18226 tests: allow small margin for unevicted pages

LU-18226 tests: allow small margin for unevicted pages

Allow a small margin for unevictable pages in case of memory pressure.

Test-Parameters: trivial
Fixes: 2a3ec2fea3 ("LU-17463 osc: add support for unevictable mlock()ed pages")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3f1cbacfdf53f217d9ebcbeb1b34693d27f70901
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56637
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Colin Faber <cfaber@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>