LU-11915 tests: fix conf-sanity 115 test

LU-11915 tests: fix conf-sanity 115 test

Not enough xattrs added to move outside inode.
Add one additional xattr. The test works only with FLAKEY=false.

Signed-off-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Test-Parameters: testlist=conf-sanity env="ONLY=115"
HPE-bug-id: LUS-6966
Change-Id: Iab13ed3434effb03e1209755ac51eba2debe7387
Reviewed-on: https://review.whamcloud.com/38849
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>