Keep only external links checks

[veggiemonk]

fixes #908

[mashb1t]

@veggiemonk @agebhar1

The last two commits contain the following changes:

• move tests to subfolder ./tests
• remove some not needed code in tests/pull_request.js
• add tests/all.js for running tests with GitHub token locally via npm run test
• move duplicate methods to helper.js

It would be great if you could give me feedback, especially on the naming of the files.

[veggiemonk]

I'll have a look 😄

[veggiemonk]

Thank you @mash1t and @agebhar1

Nice work 😄