describe('Navbar Component', () => {
  let router: Router;
  // Other definitions...

  beforeEach(async(() => {
    // Configuring Testbed etc...
  }))

  describe('routes A', () => {
    beforeEach(async(() => {
      router = TestBed.get(Router)
      router.navigate(['/', 'a']);
    }))

    it('should show the relevant links', async(() => {
      // Testing code
    }))
  })
})

this.router.events
.subscribe((routerEvent: Event) => {
  if (routerEvent instanceof NavigationStart) {
    // Do stuff
  }
});