add TDD integration tests + GHA

[gmanchon]

the goal is to make sure that whenever we merge some PR during the refacto it does not affect the correct functionality of challengify in production

benefits of TDD :

• forces to define the features developed on because they are tested
• forces to formalise the code interface
• super cool safety net to put in production

challengify

integration test : challengify_ignore files are taken into account

integration test : delimiters are correctly handled per language

integration test : challenge file and directory structure gets generated at the correct location

integration test : file deletion is handled

challengify iterate

integration test : challenge versions are generated at the correct location

integration test : file presence into a challenge version is handled correctly through challengify_iterate.yaml with to / for / from

integration test : ability to provide challenge version ranges for the presence of a file << specification needed

integration test : handle custom files like README_api.md across the challenge versions

integration test : handle presence of block of code according to only to / for / from version delimiters # ONLY_FROM_api_BEGIN

integration test : handle block of code challengification according to only to / for / from version delimiters # CHA_ONLY_FROM_api_BEGIN