ActionController::TestCase::RaiseActionExceptions

When the request.remote_addr remains the default for testing, which is 0.0.0.0, the exception is simply raised inline (skipping the regular exception handling from rescue_action). If the request.remote_addr is anything else, the regular rescue_action process takes place. This means you can test your rescue_action code by setting remote_addr to something else than 0.0.0.0.

The exception is stored in the exception accessor for further inspection.