Make embedded DERP server and TLS the default configuration for all
integration tests, replacing the per-test opt-in model that led to
inconsistent and flaky test behavior.
Infrastructure changes:
- DefaultConfigEnv() includes embedded DERP server settings
- New() auto-generates a proper CA + server TLS certificate pair
- CA cert is installed into container trust stores and returned by
GetCert() so clients and internal tools (curl) trust the server
- CreateCertificate() now returns (caCert, cert, key) instead of
discarding the CA certificate
- Add WithPublicDERP() and WithoutTLS() opt-out options
- Remove WithTLS(), WithEmbeddedDERPServerOnly(), and WithDERPAsIP()
since all their behavior is now the default or unnecessary
Test cleanup:
- Remove all redundant WithTLS/WithEmbeddedDERPServerOnly/WithDERPAsIP
calls from test files
- Give every test a unique WithTestName by parameterizing aclScenario,
sshScenario, and derpServerScenario helpers
- Add WithTestName to tests that were missing it
- Document all non-standard options with inline comments explaining
why each is needed
Updates #3139
