Add test case for listAppend in zig list bitcode

[ikelax]

This PR is related to #6301. I started to add test cases for listAppend.

Is this actually what the issue addresses? This is my first contribution and I am very new to Zig.

The PR is still a WIP because I first want to make sure that I am heading in the right direction.

[lukewilliamboswell]

Hey @ikelax thanks for checking. I think this is on track and the kind of unit tests @bhansconnect was talking about. Also, feel free to drop a post in Zulip contributing channel if you have anything to discuss, it is likely to get more eyeballs over there. I'm also happy to help review anything you have when you're ready.

I would suggest you make any changes in src/builtins for the new zig compiler and not in crates/compiler/builtins/bitcode/src.

There are two versions of the zig builtins in the compiler (running on different versions of zig), and this is the old rust implementation, which isn't being actively developed any longer. We are keeping it here so the current rust compiler builds and we can fix any high priority issues should they arise.

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