LU-18582 tests: fix conf-sanity/82b random OST idx

LU-18582 tests: fix conf-sanity/82b random OST idx

Sometimes (RANDOM * 2) % 65532 gives the
same results in a small selection. This
caused smaller number OSTs than requested
and resulted in a following failure:

 wait_update pool_list lustre.test_82b failed

Test-Parameters: trivial testlist=conf-sanity env=ONLY=82b
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: Iebaf2fedd52803eb918b4ee9c8111537930a6e38
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57531
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>