Add an integration test for --elastic-agent-cert-key-passphrase with Elastc Defend installed

[AndersonQ]

In order to fully validate #4515 a integration test should be added. The integration test should run the Elastic Agent using mTLS for control plane communication, either through a proxy or directly connecting to fleet-server.
The test needs to validate:

• the Elastic Agent is running and successfully communicating with fleet-server
• a client certificate in the policy takes precedence over the CLI configuration (--elastic-agent-cert-key-passphrase). That means, a policy client TLS configuration fully overrides the one coming from the CLI, the cert-key-passphrase is cleared if the CLI define it but not the policy
• a policy with no client TLS configuration does not override the CLI configuration
• Elastic Defend is installed and successfully connecting to fleet-server
  • open question: is relying on Elastic Defend status enough to validate that?

This issue is blocked by:

• Improve test proxy/mock fleet server to proxy HTTPS connections #4903

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)