Improve test coverage

[BomBardyGamer]

Description

We use a plugin called PIT to generate test mutation results, which shows me that the current report for the API has the following stats:

• Line coverage: 90%
• Mutation coverage: 73%
• Test strength: 98%

Which are very good statistics, but I believe we should introduce some testing further than just unit tests, possibly some integration testing, and we also need to do some acceptance and compatibility testing.

Furthermore, the statistics for the server on the other hand, are rather poor:

• Line coverage: 4%
• Mutation coverage: 2%
• Test strength: 79%

This is due to my primary method of testing being manually starting the software, which is required, but I believe we can test much more rigorously than this.

[BomBardyGamer]

I also think we should look in to better integration testing, like what Minestom has implemented, to help reduce the amount of manual testing I have to perform when testing this, which is a hell of a lot, and most of the time, I find myself skipping most of the testing, or even all of it sometimes, as I do not want to manually test.