LU-14027 tests: Fix test_135 of replay-single

LU-14027 tests: Fix test_135 of replay-single

The test_135 ("Server failure in lock replay phase") of replay-single
has an error on the method to get the pid of a background process.

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Fixes: 7ca495ec67 ("LU-14027 ldlm: Do not hang if recovery restarted during lock replay")
Test-Parameters: trivial
Test-Parameters: testlist=replay-single env=ONLY=135,ONLY_REPEAT=50
Test-Parameters: testlist=replay-single env=ONLY_REPEAT=20,ONLY=135,FAILURE_MODE=HARD clientcount=4 mdtcount=1 mdscount=2 osscount=2 austeroptions=-R failover=true iscsi=1 testlist=replay-single
Change-Id: I6ed41d75f4cbba796e39288bad8895ee1c24459f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/41227
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>