要解决“Angular单元测试 - 模拟REST服务调用”问题，可以按照以下步骤进行：

1. 创建一个Angular服务，该服务将进行REST服务调用。例如，创建一个名为RestService的服务。

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

@Injectable({
  providedIn: 'root'
})
export class RestService {
  constructor(private http: HttpClient) {}

  getData() {
    return this.http.get('/api/data');
  }

  postData(data: any) {
    return this.http.post('/api/data', data);
  }
}

2. 在组件中使用RestService。例如，在一个名为ExampleComponent的组件中调用getData和postData方法。

import { Component } from '@angular/core';
import { RestService } from './rest.service';

@Component({
  selector: 'app-example',
  template: `
    Get Data
    Post Data
  `
})
export class ExampleComponent {
  constructor(private restService: RestService) {}

  getData() {
    this.restService.getData().subscribe((data: any) => {
      console.log(data);
    });
  }

  postData() {
    const data = { name: 'John Doe' };
    this.restService.postData(data).subscribe((response: any) => {
      console.log(response);
    });
  }
}

3. 创建一个RestService的测试文件，并使用HttpClientTestingModule来模拟REST服务调用。例如，创建一个名为rest.service.spec.ts的文件。

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

describe('RestService', () => {
  let httpTestingController: HttpTestingController;
  let restService: RestService;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [ HttpClientTestingModule ],
      providers: [ RestService ]
    });

    httpTestingController = TestBed.inject(HttpTestingController);
    restService = TestBed.inject(RestService);
  });

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

  it('should get data', () => {
    const testData = { id: 1, name: 'Test Data' };

    restService.getData().subscribe((data: any) => {
      expect(data).toEqual(testData);
    });

    const req = httpTestingController.expectOne('/api/data');
    expect(req.request.method).toBe('GET');
    req.flush(testData);
  });

  it('should post data', () => {
    const testData = { name: 'John Doe' };

    restService.postData(testData).subscribe((response: any) => {
      expect(response).toBeTruthy();
    });

    const req = httpTestingController.expectOne('/api/data');
    expect(req.request.method).toBe('POST');
    expect(req.request.body).toEqual(testData);
    req.flush({});
  });
});

在以上代码示例中，我们使用HttpClientTestingModule来模拟HTTP请求和响应。HttpTestingController用于捕获HTTP请求，并进行验证。我们使用expectOne方法来检查是否有一个请求匹配了指定的URL，并使用flush方法来返回模拟的HTTP响应。在每个测试之后，我们使用verify方法来验证是否有剩余的未处理的请求。

通过按照上述步骤，您就可以进行Angular单元测试 - 模拟REST服务调用。