Expand test suite with fuzz testing

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

We should try and add fuzz testing in important areas where we take user input of some kind. This is super helpful in trying to minimize crashes and find bugs for cases that we have missed or not considered. It is also good from a security perspective.

Is it possible to construct a solution with the existing API?

Yes. Fuzz testing is built into the testing package since Go 1.18 or something like that.

Describe the solution you'd like to see.

• Investigate which sources of user input are the most important to fuzz.
• Implement fuzzing tests for these parts.
• Fix any issues that come up.
• Implement a CI workflow that runs fuzzing at some interval (i.e. not on every push or PR).