Improve coverage for BTF generation

[qjerome]

Before merging feature/fix-di branch into main, tests needs to be implemented.

For each *relevant type in https://doc.rust-lang.org/reference/types.html and their *relevant combinations:

• test proper BTF fn prototype emission
• test proper BTF struct/enum emission

*relevant type: no need to cover all types
*relevant combination: no need to cover all combinations

Topic discussed: https://discord.com/channels/855676609003651072/1026937450624450652/1160941939965841509