refactor: replace deprecated testing class

[mehmetcanay]

Pull Request

Description
HttpClientTestingModule is deprecated. Updated the tests accordingly.

Related Issue
closes #127

Type of Change

• Bugfix
• New Feature
• Documentation
• Refactoring
• Other (please specify):

Proposed Changes
Instead of HttpClientTestingModule:

import { provideHttpClient } from '@angular/common/http';
import {
  provideHttpClientTesting,
  HttpTestingController,
} from '@angular/common/http/testing';

include provideHttpClient() and provideHttpClientTesting() in providers, respectively.

How to Test
ng test

Checklist

• My code follows the project's coding style.
• I have tested my changes locally.
• I have updated the documentation (if applicable).
• I have added tests to cover my changes (if applicable).