void performRequest(Request req, Spec spec) {
        clock.advance(Duration.ofMillis(10));
        sup.connect(spec).invokeSync(req, Duration.ofSeconds(10));
    }
Tester for running rpc server.

@author Ulf Lilleengen
configserver/src/test/java/com/yahoo/vespa/config/server/rpc/RpcTester.java