Fixing Tunnelbroker tests build errors on some of the tests due to the changes in the `ConfigManager.load()` function introduced in D4129.
This diff fixes that and loads the config file depending on the mode: sandbox/no-sandbox same as we do [[ https://github.com/CommE2E/comm/blob/095e619020e36830f5eeb353b7bfc8aaa5cdf664/services/tunnelbroker/src/server.cpp#L55-L61 | in the server main ]].