Test Credential Isolation: Dedicated User Against Runtime Property Overrides in Ceylon

The objective is to execute a Ceylon integration test that resolves exclusively test-scoped credentials, avoiding any reference to production credential stores.

Ceylon's module system provides no built-in credential-substitution mechanism, requiring developers to manually configure test implementations. Additionally, during classloading, the same code may resolve production credential classes if the test classpath does not isolate the test module, leading to nondeterministic behavior.

Should the implementation rely on a dedicated test user with limited permissions and a separate credential repository, or on runtime overrides via environment variables and property placeholders? Does the test classpath guarantee isolation from production credential classes during classloading? What is the documented Ceylon mechanism for enforcing test-specific credential resolution across modules?