tests: Use Junit5 instead of Kotest

[Distractic]

Currently, all tests are made using Kotest. However, it's really hard to understand why is the error in an IDE.
In addition, some basically feature are not available to custom test (before, after, etc.).

One of advantage of Kotest is the possibility to make test in multiplateforme environment. Minestom is built with Java, so Kstom cannot be compiled in JS etc.. in this context, it will be more efficient to take Junit 5 who has lot of feature to create and custom test.

Moreover, Minestom has a test framework that will be published in a new module in few time. This framework is only usable with Junit.

In Minestom repository, I create an issue to ask the publication of the framework

[Distractic]

Here you can find an example of using the test framework