E3-286: refactor tests

As requested in https://gitlab.esss.lu.se/e3/e3-require/-/merge_requests/73, I have split this into two merge requests: This first one that refactors the testing, and then that one (which should be merged after this) which actually adds a new test and new functionality.

Notable:

• Replaced the wrapper fixture that existed before (which was simply a pathlib.Path object pointing to the wrapper) with an actual class that contains the path and a randomly generated name, as well as functionality to modify the wrapper in some specified ways.
• Added a wrappers fixture that can be used to generate multiple wrappers in the same test if need be, since this is necessary for testing dependencies between modules.
• Updated black in pre-commit config.