Add tests to the tournament module

[cocanard]

Description

This PR aim to improve the codebase coverage on the tournament module by testing new/uncovered part of the code, especially regarding matchs which aren't really tested.
Related to issue #163, not sure if it will be enough to close though.

Checklist

• I have tested the changes locally and they work as expected.
• I have added tests to cover my changes. (lol)
• I have updated the documentation accordingly.
• I have assigned the pull request to the appropriate reviewer(s).
• I have added labels to the pull request, if necessary.

[cocanard]

So on that, I'm planning to focus next on testing match related classes (groups, brackets and swiss_round) and then implement the tests I've listed so far before declaring it as ready.
The thing is, I'm not sure all of these tests are really necessary (for example endpoint tests) or on the contrary I might miss obvious use cases, as I'm still not that familiar with the codebase and primarily looking at the code coverage here.
So some insights on what's been done on this branch so far would be great.