要测试Angular HTTP拦截器中更改window.location.href值的代码，可以按照以下步骤进行：

1. 创建一个HTTP拦截器服务，该服务将拦截请求并更改window.location.href值。可以使用Angular的HttpClient模块来创建HTTP拦截器服务。

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class MyInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler): Observable> {
    // 在这里修改window.location.href值
    window.location.href = 'https://example.com';

    // 将请求传递给下一个拦截器或处理程序
    return next.handle(req);
  }
}

2. 在Angular模块中将该拦截器添加到提供程序列表中。

import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';

import { MyInterceptor } from './my-interceptor';

@NgModule({
  imports: [
    HttpClientModule
  ],
  providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: MyInterceptor,
      multi: true
    }
  ]
})
export class AppModule { }

3. 创建一个测试用例，以确保HTTP拦截器可以成功更改window.location.href值。

import { TestBed } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';

import { HttpClient } from '@angular/common/http';

describe('MyInterceptor', () => {
  let httpClient: HttpClient;
  let httpTestingController: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [ HttpClientTestingModule ],
      providers: [
        // 注入HttpClientTestingModule和HTTP_INTERCEPTORS
      ]
    });

    httpClient = TestBed.inject(HttpClient);
    httpTestingController = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    httpTestingController.verify();
  });

  it('should change window.location.href', () => {
    // 发起一个HTTP请求
    httpClient.get('/api/data').subscribe();

    // 断言window.location.href已经被更改
    expect(window.location.href).toBe('https://example.com');

    // 检查是否有对应的HTTP请求
    const req = httpTestingController.expectOne('/api/data');
    req.flush({});
  });
});

4. 运行测试用例，确保HTTP拦截器成功更改了window.location.href值。

以上是一个基本的示例，用于测试Angular HTTP拦截器中更改window.location.href值的代码。根据具体的需求，你可能需要进一步调整和扩展这个示例。