You should use now RouterTestingHarness ( https://angular.dev/api/router/testing/RouterTestingHarness ) since Angular 15.2 as RouterTestingModule is deprecated ( https://angular.dev/api/router/testing/RouterTestingModule )

see the video made by Rainer Hahnekamp (12 minutes) : https://www.youtube.com/watch?v=DsOxW9TKroo ( from https://www.rainerhahnekamp.com/en/how-do-i-test-using-the-routertestingharness/ )

The production code (you need to test the ngOnInit() method) :

ngOnInit(): void {
    if (this.router.url.endsWith('admin/user-list')) {
      this.activeLinkIndex = 1;
    } else if (this.router.url.endsWith('admin/group-list')) {
      this.activeLinkIndex = 2;
    }
  }

The test code :

import { RouterTestingHarness } from '@angular/router/testing';

describe('MyComponent', () => {
  ...

  const routes: Routes = [
    { path: 'admin/user-list', component: UserListComponent },
    { path: 'admin/group-list', component: GroupListComponent },
  ];

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [MyComponent, ... ],
      providers: [provideRouter(routes)],
    }).compileComponents();
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;

    fixture.detectChanges();
   });

...
 

    it('should set activeLinkIndex to 2 when targetted URL is admin/group-list', async () => {
            
      await RouterTestingHarness.create('admin/group-list'); 

      component.ngOnInit();

      expect(component.activeLinkIndex).toBe(2);
    });

...