fix(node-nest-starter-2020-04-03): backend test commands fail

### Problem
The test commands for the `backend` like
```sh
npm test
npm run test:e2e
```
fail due to different reasons:
1. `npm test` due to the DI cannot be resolved:
```sh
ts-jest[versions] (WARN) Version 25.1.0 of jest installed has not been tested with ts-jest. If you're experiencing issues, consider using a supported version (>=22.0.0 <24.0.0). Please do not report issues in ts-jest if you are using unsupported versions.
FAIL src/app.controller.spec.ts
AppController
root
✕ should return "API is up and running!" (14ms)

● AppController › root › should return "API is up and running!"

Nest can't resolve dependencies of the AppController (AppService, ?). Please make sure that the argument at index [1] is available in the _RootTestModule context.

at Injector.lookupComponentInExports (../node_modules/@nestjs/core/injector/injector.js:180:19)

● AppController › root › should return "API is up and running!"

TypeError: Cannot read property 'healthCheck' of undefined

23 | describe('root', () => {
24 | it('should return "API is up and running!"', () => {
> 25 | expect(appController.healthCheck()).toBe('API is up and running!');
| ^
26 | });
27 | });
28 | });

at Object.it (app.controller.spec.ts:25:28)

Test Suites: 1 failed, 1 total
Tests: 1 failed, 1 total
Snapshots: 0 total
Time: 2.155s, estimated 3s
```

2. `npm run test:e2e` it cannot connect to the database although in `npm start run:dev` things work fine. I run it as described in #64
```sh
> jest --config ./test/jest-e2e.json

ts-jest[versions] (WARN) Version 25.1.0 of jest installed has not been
tested with ts-jest. If you're experiencing issues, consider using a su
pported version (>=22.0.0 <24.0.0). Please do not report issues in ts-jest if you are using unsupported versions.
[Nest] 94172 - 28/04/2020, 12:17 [MongooseModule] Unable to connect
to the database. Retrying (1)...
[Nest] 94172 - 28/04/2020, 12:17 [MongooseModule] Unable to connect
to the database. Retrying (2)... +3007ms
FAIL test/app.e2e-spec.ts (7.169s)
AppController (e2e)
✕ / (GET) (5008ms)

● AppController (e2e) › / (GET)

Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Error: Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.
```
- Generally, I cannot find the expected string `'Hello World!'` elsewhere in the code, and after trying the nest starters it seems like a copy-paster from the scaffolder.
- Could you provide additional help to set up the testing properly? I'd like to develop in TDD style