Add a GitHub Action that comments on pull requests if tests are missing

[oliverklee]

https://github.com/infection/tests-checker-action

[hemanthhhhhh]

Please assign this issue to me

[oliverklee]

Hi @hemanthhhhhh, @ghostwriter already has taken this issue. Unfortunately, GitHub only allows assigning issues to project members and not to outside contributors. Sorry. :-(

[oliverklee]

Fixed by #730.