keep test input & output files

[VladimirAlexiev]

https://github.com/statnett/KGraphPy/tree/main/tests includes only source.

• I see in https://github.com/statnett/KGraphPy/blob/main/main.py#L55 that certain input files are being converted
• please save in test the input (eg CIMXML, SHACL) & output (eg JSONLD, Trig, ValidationReports) files, so people can examine them
  ... without having to look at Python source