LU-2283 test: fix sanity-quota test_18

LU-2283 test: fix sanity-quota test_18

test_18 should grep the watchdog message start from "sanity-quota
test 18" instead of "test 18b", otherwise, the messages from other
tests could be mis-processed.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Icf4c13cd5f602cbb1974fc3ca5d156c87398cd3e
Reviewed-on: http://review.whamcloud.com/4484
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>