Defined test files in a StrEnum

Instead of defining the path every time that a file is to be called, I created a StrEnum with the paths of all needed (used) files. The test changes are minimal and the only deprecations are due to duplicated code.

Important note: The StrEnum was actually introduced in python 3.11, so to be able to use the tests in previous versions I installed the PyBackport package as a dependency only for the tests.