Speed up your component's test suite by caching the test module

app.component.spec.ts

import { TestBed, async } from '@angular/core/testing';
import { AppComponent } from './app.component';
import { setUpCachedTestBed } from '../test-utils';

describe('AppComponent', () => {
  setUpCachedTestBed({
      declarations: [ AppComponent ],
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(AppComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create the app', async(() => {
    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app).toBeTruthy();
  }));
});

test-utils.ts

import { TestBed, TestModuleMetadata } from '@angular/core/testing';

const resetTestingModule = TestBed.resetTestingModule;

export function setUpCachedTestBed(moduleDef: TestModuleMetadata) {
  beforeAll(done => {
    TestBed.resetTestingModule();
    TestBed.resetTestingModule = () => TestBed;
    TestBed.configureTestingModule(moduleDef);
    TestBed.compileComponents()
      .then(done).catch(done.fail);
  });

  afterAll(() => TestBed.resetTestingModule = resetTestingModule);
};

Based on angular/angular#12409