在Angular的单元测试中，如果出现"$未定义"的错误，通常是因为测试中没有正确引入Angular的相关依赖。以下是可能的解决方法：

1. 确保在测试文件的开头处正确引入了Angular的相关依赖：

import { TestBed } from '@angular/core/testing';
import { HttpClientTestingModule } from '@angular/common/http/testing';
import { YourComponent } from './your.component';

describe('YourComponent', () => {
  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [YourComponent],
      imports: [HttpClientTestingModule],
    }).compileComponents();
  });

  it('should create the component', () => {
    const fixture = TestBed.createComponent(YourComponent);
    const component = fixture.componentInstance;
    expect(component).toBeTruthy();
  });
});

2. 确保在测试文件中正确注入了Angular的服务或组件：

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { YourService } from './your.service';
import { YourComponent } from './your.component';

describe('YourComponent', () => {
  let component: YourComponent;
  let fixture: ComponentFixture;
  let yourService: YourService;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [YourComponent],
      providers: [YourService],
    }).compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(YourComponent);
    component = fixture.componentInstance;
    yourService = TestBed.inject(YourService); // 正确注入服务
  });

  it('should create the component', () => {
    expect(component).toBeTruthy();
  });
});

3. 如果在测试文件中使用了Angular的内置指令（如ngIf、ngFor等），确保在测试之前正确编译组件模板：

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { YourComponent } from './your.component';

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

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [YourComponent],
    }).compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(YourComponent);
    component = fixture.componentInstance;
    fixture.detectChanges(); // 编译组件模板
  });

  it('should create the component', () => {
    expect(component).toBeTruthy();
  });
});

希望以上解决方法能够帮助您解决"Angular单元测试错误（$未定义）"的问题。