System test CI is missing agent error and never quits

Thanks to @marc-gr for catching this one, it has been causing headaches for me as well, and his pointers help me find the issues in my PR's.

When certain errors happens in agent.log while doing system tests, they are not caught and one out of 2 things will happen:
1. System tests both locally and on CI will just hang/loop until jenkins times out.
2. System test CI will fail on the step "Running elastic-package stack down".

Both of these does not really show what the error is, and it is quite hard to troubleshoot. However Marc managed to find this log line:
`2021-06-03T14:29:40.250Z ERROR fleet/fleet_gateway.go:180 failed to dispatch actions, error: fail to generate program configuration: failed to add stream processor to configuration: InjectStreamProcessorRule: processors is not a list`

While this error is specific to how we wanted to add processors to the `hbs` files, and there was a chance that processors was `null`, I feel that system tests should catch any issues with generating configuration and instantly fail/stop the CI, as this will go on for a LONG time until failing.