test: check validity of systemd service files
This spots errors like what commit a5314584 ('cloud-setup: fix placement of directives in the systemd unit') fixes.
This is what a failure would look like:
5/74 check-nm-cloud-setup.service FAIL 0.05s exit status 1
... /bin/bash src/tests/check-systemd-service.sh build/src/nm-cloud-setup/nm-cloud-setup.service ―――――――――――――――――――――― ✀ ――――――――――――――――――――――― build/src/nm-cloud-setup/nm-cloud-setup.service:19: Unknown key 'StartLimitIntervalSec' in section [Service], ignoring.
https://issues.redhat.com/browse/RHEL-56860 https://issues.redhat.com/browse/RHEL-49694