fix test that checks if systemd has failed units

[Vincinator]

The features/base/test/test_systemd test does not detect (all) failed units. Currently, if we manually verify on a booted GL image, we get the following failed units:

• rsyslog
• syslog
• fail2ban

[Vincinator]

Maybe this is a timing issue. Test could run before the first units have failed.

• This test runs on nightly platform tests see
• This test does not run on chroot target, so if you execute ./test kvm-pyton_dev it will not run.
• I can reproduce the issue. I build a kvm-python_dev, started it and have failed systemd units. I opened a python3 interpreter, and executed the test steps manually, to see if the test logic is broken. Manually stepping through a copy-paste (with adaptions) version of the test works as expected and detects the failed units.

=> I suspect that this is a timing issue. The tests executed as part of the platform tests is executed before the system is fully booted, so that the faulty units have not failed yet.