Testing: Move all datasets into conftest.py

[eroell]

Description of feature

The datasets for testing are currently distributed within the specific tests.

A less redundant, clearer and easier to do maintenance solution is to have them as fixtures in conftest.py.

TODOs

• [ ] Collect what datasets are used in testing
• [ ] Evaluate where to store them & if redundancies can be removed
• [ ] Write fixtures for dataset loading in conftest.py that replace individual dataset loading in the test scripts