LU-15166 tests: restore osp-syn threads after test_818

test_818() is supposed to leave osp-syn threads up after the test end,
otherwise, following tests get "logging isn't available, run LFSCK".

Use fail $SINGLEMDS for that.

Test-Parameters: trivial testlist=sanity
HPE-bug-id: LUS-10495
Change-Id: Ib4876f4c4d39fc87f86788d8611838b8078e4aac
Signed-off-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Reviewed-on: https://review.whamcloud.com/45375
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>