LU-4671 tests: give multiop a chance to exit.

LU-4671 tests: give multiop a chance to exit.

If 'multiop' is still running after a test complete, test-framework.sh
reports a failure.
test_43A signals multiop asking it to exit as the last thing it does.
If there is any delay in multip being schedule, test-framework will
see it and report an error - this is a false negative.

So use 'wait' to wait for multiop to respond to the signal.

Test-Parameters: trivial testlist=sanity envdefinitions=ONLY="43A"
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic94d3c89bd98c9f6ef2d5bee22aac1e39116bf11
Reviewed-on: https://review.whamcloud.com/39665
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>