test apm-server package when running in unprivileged mode by Elastic Agent

[simitt]

The Elastic Agent capabilities are extended to allow running packages in unprivileged mode (currently experimental). Integrations that do require root privileges need to update the package spec to set agent.privileges.root: true. APM Server does not need to run with privileges, therefore there are no plans for updating apm package.

This issue is to test functionality when running in the newly supported unprivileged mode. It can be tested by starting the agent with an --unprivileged flag, see elastic/integrations#8642 (comment).