LU-12350 tests: Do not use background failover

LU-12350 tests: Do not use background failover

For some reason, test 33 chooses at one point to take an
OST offline by starting failover in the background. It
seems to assume the OST will be offline during the
subsequent read, without doing anything to verify it is
offline - In fact, it could either be not offline yet or
back online with failover complete.

Just use stop like the rest of the test does.

Lustre-change: https://review.whamcloud.com/34985
Lustre-commit: 4ac0324fb9d824915b3dd11b75e81e609d9e8e84

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I9c074ff1412793b8f0d8f15dc1e2ee21bb6d9fd6
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35086
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>