Add start of a router_test module for hooking in the new routing system.

core/modules/system/tests/modules/router_test/router_test.info

+name = "Router test"
+description = "Support module for routing testing."
+package = Testing
+version = VERSION
+core = 8.x
+hidden = TRUE

core/modules/system/tests/modules/router_test/router_test.module

+<?php
+
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+function router_test_router_info() {
+  $collection = new RouteCollection();
+
+  $route = new Route('router_test/test1');
+  $collection->add('router_test_1', $route);
+
+  return $collection;
+}