Stray grep warnings

[seowwj]

Issue: There are some stray grep warnings when the test has ended:

Originally posted by @seowwj in #483 (comment)

Reproduction steps:

image=amazonlinux/2023
hostname=hostname=lkp-docker.${image//\//.}

lkp docker build --image $image --hostname $hostname
lkp docker test -i $image -j schbench.yaml --any --hostname $hostname