Datasetmanager tests race condition

[rettigl]

DatasetManager tests fail some times if executed concurrently. This is because all tests access the same global dataset configuration. Also, running the tests leaves a "test" folder behind. This should be avoided by properly mocking paths, and using tmp_path.

[zain-sohail]

@Aserhisham Potentially a simpler first issue to tackle compared to writing lab loader tests. I tried to fix it in the PR #562 but the race conditions are still there. If you prefer, you can ignore my changes and work from main branch, or build on top of the PR.

[rettigl]

I recently learned there is a convenient way to avoid this by using load groups. Using this approach you can place all tests that use similar resources into the same worker thread. See e.g. here: FAIRmat-NFDI/pynxtools#626