private static ProxyServer createTestServer(ConfigSourceSet source, ConfigSourceClient configSourceClient) {
        return new ProxyServer(null, source, configSourceClient);
    }
Verifies that config with generation 0 (used for empty sentinel config) is not cached.
If it was cached, it would be served even when newer config is available
(as they ask for config, saying that it now has config with generation 0).
When the config has been successfully retrieved it must be updated in cache.
config-proxy/src/test/java/com/yahoo/vespa/config/proxy/ProxyServerTest.java