title: How to overrideProvider for NestJS/TypeORM
body: <p><strong>api.controller.ts</strong></p>
  <pre><code>@Controller('api')
  export class ApiController {
    constructor() {}
  
    @Post()
    @Transaction()
    async root(@Req() req: Request, @Res() res: Response, @TransactionManager() manager: EntityManager): Promise&lt;void&gt; {
      res.send(/* anything */);
    }
  }
  </code></pre>
  <p><strong>api.e2e-spec.ts</strong></p>
  <pre><code>describe('API (e2e)', () =&gt; {
    let app: INestApplication;
    let connection: Connection;
  
    beforeAll(async () =&gt; {
      const module = await Test.createTestingModule({
        imports: [],
        controllers: [ApiController],
        providers: [],
      })
      .overrideProvider('Connection')
      .useValue(/** How to ??? */)
      .compile();
      app = module.createNestApplication();
      await app.init();
    });
  });
  </code></pre>
  <p>Result is</p>
  <pre><code>[Nest] 35181   - 10/30/2018, 5:42:06 PM   [ExceptionHandler] 
  Connection &quot;default&quot; was not found.
  ConnectionNotFoundError: Connection &quot;default&quot; was not found.
  </code></pre>
  <p>In this case, I just want to check request parameters and response values using mock by test.
  I'd like to disable the DB connection by override it.</p>