可以使用Angular提供的FormControl和FormGroup来测试输入字段是否正确地更改了值。我们可以创建一个FormGroup并将FormControl绑定到输入字段上，在测试中，我们可以模拟外部输入并通过断言来验证输入字段是否正确更新。示例代码：

import { async, ComponentFixture, TestBed } from '@angular/core/testing';
import { ReactiveFormsModule, FormGroup, FormControl } from '@angular/forms';

import { MyComponent } from './my.component';

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

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [ ReactiveFormsModule ],
      declarations: [ MyComponent ]
    })
    .compileComponents();
  }));

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

    // create FormGroup
    component.myForm = new FormGroup({
      myInput: new FormControl('')
    });
  });

  it('should change input field value correctly based on external input', () => {
    // simulate external input
    component.myForm.get('myInput').setValue('New value');

    // assert input field value
    const inputElement = fixture.nativeElement.querySelector('input');
    expect(inputElement.value).toEqual('New value');
  });
});