chore(v2): Implement a simple E2E testing mechanism for docusausus start

[SamChou19815]

Motivation

Implement a simple mechanism to E2E test docusausus start, so that we can catch no-prod only issues at diff time (avoid things like #2944). It uses the done webpack compiler hook to ensure that webpack can compile without errors. In this way, we can completely rule out disasters like #2883.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Run E2E_TEST yarn start --no-open on the following scanarios:

1. Pass with no issues

2. Fail when we have a bad client module.

3. Fail when we have some client syntax errors.

4. Fail if we cannot load plugin from docusaurus.config.js

5. Fail if we cannot load plugin from preset.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[docusaurus-bot]

Deploy preview for docusaurus-2 ready!

Built with commit e78f830

https://deploy-preview-2950--docusaurus-2.netlify.app

[SamChou19815]

Looks like the new integration test is already catching some problems. See https://github.com/facebook/docusaurus/pull/2950/checks?check_run_id=778802180. #2884 introduced some nullish coalescing syntax that the current babel loader doesn't like.

[slorber]

LGTM

Hey, wasn't aware of this "test-website" e2e tests.

Wonder if it's not worth it to do the same for the bootstrap theme as well? @fanny

[fanny]

Interesting, I think that would be great, I'll work on