Add daemon default Unix socket TLS integration test

[taniwha3]

Depends on #591; please merge that first.

Summary

• add an integration test that starts auraed in default mode, waits for the runtime Unix socket, and verifies TLS gRPC discovery succeeds
• expose a mutable child accessor in the test ChildGuard used by the new test

Testing

• manual: auraed_daemon_default_should_bind_runtime_unix_socket_and_accept_grpc

[dmah42]

generally. I try to avoid integration tests as much as possible, only because they can be flaky and hard to maintain. this seems reasonable but let's try to keep few of them.