Make the component test to have a body or mock Renderer2

Other approach would be, mocking service:

let renderer: MockRenderer;
class MockRenderer {
   addClass(document: string, cssClass: string): boolean {
     return true;
   }
}

beforeEach(async( () => {
   TestBed.configureTestingModule({
    ...
       providers: [{
              provide: Renderer2,
              useClass: MockRenderer
            }]
    }).compileComponents().then(() => {
        fixture = TestBed.createComponent(YourComponent);
        renderer =  fixture.debugElement.injector.get(Renderer2);
    });
}));

...
it('should call render', () => {
      spyOn(renderer, 'addClass');
      ...
      expect(renderer.addClass).toHaveBeenCalledWith(jasmine.any(Object), 'css-class');
});

Instead of mocking the renderer try to hijack it. This should be working with Angular 6+.

In your component.spec.ts:

let renderer2: Renderer2;
...
beforeEach(async( () => {
    TestBed.configureTestingModule({
        ...
        providers: [Renderer2]
    }).compileComponents();
}));

beforeEach(() => {
    fixture = TestBed.createComponent(YourComponent);
    // grab the renderer
    renderer2 = fixture.componentRef.injector.get<Renderer2>(Renderer2 as Type<Renderer2>);
    // and spy on it
    spyOn(renderer2, 'addClass').and.callThrough();
    // or replace
    // spyOn(renderer2, 'addClass').and.callFake(..);
    // etc
});

it('should call renderer', () => {
    expect(renderer2.addClass).toHaveBeenCalledWith(jasmine.any(Object), 'css-class');
});