[ty] Add tests for variable-length tuple iteration

[MatthewMckee4]

Summary

Resolves astral-sh/ty#2202.

Both of the following snippets are still not covered.

I'm not sure if these are completely unreachable, or I just can't think of a case to test them.

But the two new added tests add more coverage.

(TupleSpecBuilder::Fixed(_), TupleSpec::Fixed(_)) => None,

.or_else(|| {
    self_built.resize(db, var.len()).ok().and_then(|resized| {
        TupleSpecBuilder::from(&resized).intersect(db, other)
    })
})

Test Plan

cargo nextest run -p ty_python_semantic -- mdtest::loops

[sharkdp]

Thank you!

I'm not sure if these are completely unreachable, or I just can't think of a case to test them.

I believe they are unreachable at the moment, but it looks to me like it doesn't hurt to leave the "dead" code in place (as opposed to adding unreachable!(…) or similar), just in case the function gets called in a different way in the future.

[AlexWaygood]

I think that's correct. I suppose we could add test coverage for the unreachable branches with a Rust unit test in tuple.rs, but it might not be worth it

[AlexWaygood]

It would also be great if we could add tests for how these intersections of tuples behave when they're unpacked. When we have a test that just uses them in a for loop, all we see is the union of all the elements in the tuple spec