Remove classes in tests

[lionelkusch]

It's the case --- as we have said many times. Nevertheless, it's fine to handle this in a future PR.

Originally posted by @bthirion in #317 (comment)

[lionelkusch]

I used classes because this allows grouping the test with themes. When there are some problems, I can run only this group of tests without launching all the tests on the file through the interface of vscode/IDE. It's very helpful and reduces the time to wait before getting the results of the tests.
If you know a way to run only a specific group of tests through the interface of vscode/IDE, I will be very happy to use it and remove the classes from the tests.
The group of test is base actually on the management of tests by pytest.

[bthirion]

You have good points, but the price to pay is that it makes it complex for new contributors to amend, improve or develop tests.
Having a flat test structure minimizes the cognitive burden for contributors.

[lionelkusch]

When there are more than 20 tests, having an organisation on top of it can help not to be lost between them.