Reorganize/refactor tests

[mpangrazzi]

We need to reorganize/refactor tests since it's a bit confusing at the moment.

Here's a brief list of some of the tasks we most likely need to do:

• Check if test marks are used properly
• Check if we can remove _it_ tags. It was initially used to mark integration test files (i.e. not pure unit tests). Maybe now we can reconsider this choice
• Reorganize test files, clearing unneeded ones
• Create a CONTRIBUTING.md file