Unit test fails with NullInjectorError in Angular

I'm new to Angular and I'm missing something when it comes to understanding how DI works. I'm currently trying to get a unit test to pass.

Here is the code for the test;

import { TestBed } from '@angular/core/testing';
import { HttpClientModule, HttpClient } from '@angular/common/http'
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import { InjectionToken } from '@angular/core';
import { UserService } from './user.service';

export const BASE_URL = new InjectionToken<string>('BASE_URL');

describe('UserService', () => {
  beforeEach(() => TestBed.configureTestingModule({
    imports: [
      HttpClientTestingModule,
    ],
    providers: [
      { provide: BASE_URL, useValue: 'http://localhost' },
      UserService,
      HttpClient
    ]
  }));

  it('should be created', () => {
    const service: UserService = TestBed.get(UserService);
    expect(service).toBeTruthy();
  });
});

Here is the code under test;

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

import { User } from '@/_models';

@Injectable({ providedIn: 'root' })
export class UserService {

  private baseUrl: string

  constructor(private http: HttpClient, @Inject('BASE_URL') baseUrl: string) {
    this.baseUrl = baseUrl
  }

  getAll() {
    return this.http.get<User[]>(`${this.baseUrl}/users`);
  }

  getById(id: number) {
    return this.http.get(`${this.baseUrl}/users/${id}`);
  }

  register(user: User) {
    return this.http.post(`${this.baseUrl}/users/register`, user);
  }

  update(user: User) {
    return this.http.put(`${this.baseUrl}/users/${user.id}`, user);
  }

  delete(id: number) {
    return this.http.delete(`${this.baseUrl}/users/${id}`);
  }
}

This is my error;

Error: StaticInjectorError(DynamicTestModule)[BASE_URL]:
StaticInjectorError(Platform: core)[BASE_URL]: NullInjectorError: No provider for BASE_URL!

Why am I getting this error and what do I need to do to rectify it?