testing-farm: edge-8to9-94 and edge-9to9-94 can pass but testing-farm plugin reports fail

[yih-redhat]

Descripion:

I have a pull request yih-redhat/tmt-demo#42 that runs all test cases in testing-farm with v2.
In this pull request, the sub job "Testing Farm - edge-9to9-9.4" is very strange, the test script is passed and exit with 0, but testing-farm plugin always report timetout error. Job link is https://artifacts.osci.redhat.com/testing-farm/befe8230-0cca-4417-816c-af13e20f564f/
The sub job "Testing Farm - edge-8to9-9.4" has the same issue. And in this job, I checked all leftover processes in vm that may cause the timeout bug and printed them out in log, job link is https://artifacts.osci.redhat.com/testing-farm/24f28bf8-1c7d-47d5-9779-63723ecfb222/
All sub jobs running in this pull request has same configuration. but only "Testing Farm - edge-9to9-9.4" and "Testing Farm - edge-8to9-9.4" has this strange timeout issue. Which means there might be something in the test scripts that caused this issue but not the configuration. The test script for these two sub jobs are https://github.com/yih-redhat/tmt-demo/blob/main/ostree-9-to-9.sh and https://github.com/yih-redhat/tmt-demo/blob/main/ostree-8-to-9.sh, but I cannot see anything special in these scripts, they are just normal shell scripts, like other test scripts in my repo.

The test script was actually passed and exit with 0, but it looks like some child process blocked the job to complete until timeout.
I have tried to set the timeout to a very long time, and still got this issue. And with the same timeout value, other sub jobs which take much longer than this script can pass.