LU-13147 tests: Cleanup sanity-lnet on test failure

LU-13147 tests: Cleanup sanity-lnet on test failure

Trap EXIT so we can cleanup on test failure.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I702b214046a68af2b87536dab01879c356bff2a8
Reviewed-on: https://review.whamcloud.com/37258
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>