要测试Angular中的HostBinding和CSS类，您可以按照以下步骤进行：

1. 创建一个组件，例如AppComponent：

import { Component, HostBinding } from '@angular/core';

@Component({
  selector: 'app-root',
  template: `
    

  `,
  styles: [`
    .red {
      background-color: red;
    }
  `]
})
export class AppComponent {
  @HostBinding('class.blue') isRed = true;
}

2. 创建一个测试文件，例如app.component.spec.ts，并导入必要的依赖项：

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { AppComponent } from './app.component';

describe('AppComponent', () => {
  let component: AppComponent;
  let fixture: ComponentFixture;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ AppComponent ]
    })
    .compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(AppComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should have "blue" class applied', () => {
    const hostElement: HTMLElement = fixture.nativeElement;
    expect(hostElement.classList.contains('blue')).toBeTruthy();
  });

  it('should have "red" class when isRed is true', () => {
    component.isRed = true;
    fixture.detectChanges();
    const hostElement: HTMLElement = fixture.nativeElement;
    expect(hostElement.classList.contains('red')).toBeTruthy();
  });

  it('should not have "red" class when isRed is false', () => {
    component.isRed = false;
    fixture.detectChanges();
    const hostElement: HTMLElement = fixture.nativeElement;
    expect(hostElement.classList.contains('red')).toBeFalsy();
  });
});

3. 运行测试：

ng test

这样，您就可以测试AppComponent中的HostBinding和CSS类了。第一个测试用例验证了是否应用了"blue"类，第二个和第三个测试用例验证了当isRed为true或false时，是否正确应用或移除了"red"类。