LU-16947 tests: On error correctly kill multiop

LU-16947 tests: On error correctly kill multiop

multiop_bg_pause under test-framework starts multiop
in background and waits for signal if "_" option is
provided. On 'verbose' mode the PAUSING string is
printed on console which is checked and if not found
error is reported by multiop_bg_pause function.

On error, it is required to kill the existing running
multiop binary and if not done will eventually timeout
and not exit the test.

Currently on error multiop_bg_pause function incorrectly
sends signal to wrong PID. This patch fixes this issue.

Test-Parameters: trivial testlist=replay-single mdscount=2 mdtcount=4
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I3fb505302615512a891725e7339a6f0238c2cdab
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51589
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>