elastic-package system tests should be a little more flexible, not require external service containers

This is an extension of https://github.com/elastic/integrations/pull/1358

The system/linux integrations are a bit odd in that they don't require any kind of external service container, and 90% of the data streams can be tested by just pointing metricbeat at a spoofed procfs root. As it stands now, `elastic-package test system` seems to require an external container for running a service, which is a tad inconvenient. Going forward, we should have some built-in system in the test definition that:

- doesn't require a service container to be spun up, other than the agent container that does the ingestion
- Can mount a directory onto the agent test container