tests: init

[Misterio77]

Hi! This is me again, with the first of 4 PRs this time xD

This PR adds simple NixOS integration tests, which can be run with nix flake check (or individually with nix build .#checks.x86_64-linux.simple).

For now, there's two:

• Simple: runs the vanilla server and check that it actually started
• Files & Symlinks: runs the paper server, with two nix expression-generated files (ops.json and paper.yml), as well as symlink (a vanilla jar that is nescessary to avoid paper trying to download it at runtime).
  • In a follow-up PR (will open it in a few minutes), I will use this to test command execution:
    • I de-op Misterio7x, this mutates the ops.json, so validates that files are mutable.
    • I run a unknown command and grep the output, validating that the paper.yml unknown-command is actually used.

[Infinidoge]

Looks pretty good to me. Will do local testing to make sure everything is good before merging.

Darn you Paper! Why must you download things at runtime! It might be a good idea to put the corresponding vanilla jar for a paper server into the paper derivation's passthru. Also would be a good idea to document the need for that in the readme.

[Misterio77]

For normal usage, we usually can assume the user will have internet connection, but I fully agree, we should totally add that to passthrough and document it; I'll do this in a minute.

[Misterio77]

There you go: https://github.com/Infinidoge/nix-minecraft/pull/44

[Infinidoge]

Sorry for taking so long to merge this. While I do have the excuse that I've been busy with college, it really shouldn't have taken so long to merge a PR of this size. Thanks again for the contributions!