Merge test-directory-structure test into HTML Proofer's custom checks and run it in CI

[yasulab]

This PR enables to check broken links/images/scripts/OGP/favicon and existing test-directory-structure test in one html-proofer's test report by using custom tests.

Also this PR enable to run bundle exec rake test in CI.

- name: 🔧 Build & Test
  run: |
    JEKYLL_ENV=production bundle exec jekyll build
    JEKYLL_ENV=production bundle exec jekyll doctor
    SKIP_BUILD=true       bundle exec rake test

📝 NOTE: jekyll build generates HTML so rake test can skip building it again.

Example CI report with Failure

$ bundle exec rake test

...

Running 6 checks (CustomChecks, Favicon, Images, Links, OpenGraph, Scripts) in ["_site"] on *.html files ...

	checking ... /404
	checking ... /code-of-conduct/index
	checking ... /contests/how-to-apply/index
	checking ... /contests/how-to-apply
	checking ... /contests/index
	checking ... /contests/terms/index
	checking ... /contests/terms
	checking ... /events/index
	checking ... /events/saikyo-coderdojo/index
	checking ... /index
	checking ... /news/index
	checking ... /privacy/index
	checking ... /registration/index
	checking ... /sessions/keynote1/index
	checking ... /sessions/keynote2/index
	checking ... /sponsorship/index
	checking ... /sponsorship/list/index
	checking ... /staff/index
	checking ... /test-temp

Checking 21 internal links
Checking internal link hashes in 1 file
Ran on 19 files!


For the CustomChecks check, the following failures were found:

* At _site/test-temp.html:

  index.html でない HTML ファイルが生成されました:
  File: _site/test-temp.html
  すべてのページは index.html として生成される必要があります。
  これにより /path/ と /path の両方でルーティングが可能になります。

HTML-Proofer found 1 failure!

Example CI report with Success

$ bundle exec rake test

...

	checking ... /404
	checking ... /code-of-conduct/index
	checking ... /contests/how-to-apply/index
	checking ... /contests/how-to-apply
	checking ... /contests/index
	checking ... /contests/terms/index
	checking ... /contests/terms
	checking ... /events/index
	checking ... /events/saikyo-coderdojo/index
	checking ... /index
	checking ... /news/index
	checking ... /privacy/index
	checking ... /registration/index
	checking ... /sessions/keynote1/index
	checking ... /sessions/keynote2/index
	checking ... /sponsorship/index
	checking ... /sponsorship/list/index
	checking ... /staff/index

Checking 21 internal links
Checking internal link hashes in 1 file
Ran on 18 files!

HTML-Proofer finished successfully.

[kp54]

LGTM
ありがとうございます！