It looks like you are approaching the problem in the wrong way. You should mock the method in "load-json.service.ts", the mocked method should return the exact JSON

If you are testing some components, the following is what I am suggesting.

Notice in the providers, we are forcing the component to use our mocked LoadJsonService.

...
const jsonDataObj = {foo: 'bar'};
const mockedLoadJsonService = {
  getJson:()=> JSON.stringify(jsonDataObj)
}

beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [
        ...
      ],
      declarations: [...],
      providers: [
        { provide: LoadJsonService, useValue: mockedLoadJsonService },
      ],
    }).compileComponents();
  });

...

I can give you an example from my project. Maybe it will help you to get an idea.

describe("Interviews service", () => {
let httpClientSpy: jasmine.SpyObj<HttpClient>;
let interviewsService: InterviewsService;

 beforeEach(() => {
 const apiUrlsServiceFake = jasmine.createSpyObj('ApiUrlsBuilderService', ['getApiUrl']);
 httpClientSpy = jasmine.createSpyObj('HttpClient', ['post']);
interviewsService = new InterviewsService(httpClientSpy, apiUrlsServiceFake);
});

it('should create new interview and return new interview data', (done: DoneFn) => {
const expectedNewInterviewData: NewInterviewData = { id: 1234, encodedId: "qweasd" };

httpClientSpy.post.and.returnValue(asyncData(expectedNewInterviewData));

 interviewsService.createNewInterview(1111, "New interview").subscribe({
  next: newIwData => {
    expect(newIwData).withContext("Expect new interview data").toEqual(expectedNewInterviewData);
    done();
  },
  error: done.fail
});

 expect(httpClientSpy.post.calls.count()).withContext('one call').toBe(1);   });   });