Allow negative testing in system tests

The system test equivalent of https://github.com/elastic/elastic-package/issues/912.

I'd like to assert that an error document is produced and that the agent goes into a degraded state. An example of the type of logic this could test is in https://github.com/elastic/integrations/pull/14771.

Start by reading the linked elastic-package issue 912 and the logic demonstrated in integrations pull request 14771. Identify the system-test entry points and existing assertions for agent state and output documents; done means a system test can verify both an error document and a degraded agent state.

Written by the indexing model from the issue text.