Organise tests into folders and files and create main for running all tests

[MrME-CodeSmith]

@MatjereJ and @U18034332, please create a testing environment for the Kotlin backend

Your tasks are:

• Create a folder structure for specific tests, e.g. Controller tests, Entity tests, etc.
• Create a main in the overarching tests folder to run the tests altogether

[MrME-CodeSmith]

Branch: feature/{add time frame}