Lazily setup the router in non-application tests for <LinkTo> component

PR emberjs#19080 fixed the issue where accessing public router service throwing
error, however, the PR did not support LinkTo component which uses the
internal -routing service. This PR adds similar lazy setup to
-routing.router.

packages/@ember/-internals/glimmer/tests/integration/components/link-to/rendering-angle-test.js

@@ -1,4 +1,9 @@
-import { moduleFor, ApplicationTestCase, RenderingTestCase, runTask } from 'internal-test-helpers';
+import {
+  moduleFor,
+  ApplicationTestCase,
+  RenderingTestCase,
+  runTask,
+} from 'internal-test-helpers';
 
 import Controller from '@ember/controller';
 import { set } from '@ember/-internals/metal';
@@ -101,7 +106,7 @@ moduleFor(
     ['@test should be able to be inserted in DOM when the router is not present - block']() {
       this.render(`<LinkTo @route='index'>Go to Index</LinkTo>`);
 
-      this.assertText('Go to Index');
+      this.assertComponentElement(this.element.firstChild, { tagName: 'a', attrs: { href: '#/' }, content: 'Go to Index' });
     }
   }
 );

packages/@ember/-internals/routing/lib/services/routing.ts

@@ -2,12 +2,16 @@
 @module ember
 */
 
+import { getOwner, Owner } from '@ember/-internals/owner';
 import { readOnly } from '@ember/object/computed';
 import { assign } from '@ember/polyfills';
 import Service from '@ember/service';
+import { symbol } from '@ember/-internals/utils';
 import EmberRouter, { QueryParam } from '../system/router';
 import RouterState from '../system/router_state';
 
+const ROUTER = symbol('ROUTER') as string;
+
 /**
   The Routing service is used by LinkComponent, and provides facilities for
   the component/view layer to interact with the router.
@@ -19,7 +23,17 @@ import RouterState from '../system/router_state';
   @class RoutingService
 */
 export default class RoutingService extends Service {
-  router!: EmberRouter;
+  get router(): EmberRouter {
+    let router = this[ROUTER];
+    if (router !== undefined) {
+      return router;
+    }
+    const owner = getOwner(this) as Owner;
+    router = owner.lookup('router:main') as EmberRouter;
+    router.setupRouter();
+    return (this[ROUTER] = router);
+  }
+
   hasRoute(routeName: string) {
     return this.router.hasRoute(routeName);
   }

packages/@ember/engine/index.js

@@ -511,8 +511,6 @@ function commonSetupRegistry(registry) {
 
   // Register the routing service...
   registry.register('service:-routing', RoutingService);
-  // Then inject the app router into it
-  registry.injection('service:-routing', 'router', 'router:main');
 
   // DEBUGGING
   registry.register('resolver-for-debugging:main', registry.resolver, {