Fix return value and r_valid value in Variant::iter_init and Variant::iter_next

[gilzoide]

Variant iteration methods were returning wrong values from GDExtension API call, never reporting that iterations were valid.
This PR makes r_valid be set, as well as return the result from the call instead of returning valid.

[dsnopek]

Thanks! The changes look good to me, and seem to work in my testing!

Attached is a patch that adds some automated tests to ensure that at least iter_init() is working correctly. (It's tricky to test iter_next(), because you shouldn't even be able to get to that point, if you're correctly checking r_valid on iter_init() - but your changes look correct anyway!) Unfortunately, the tests won't pass until PR godotengine/godot#83681 is fixed in Godot. So, I'll make a follow up PR later that adds these tests.

pr-1253-tests.txt

[dsnopek]

Thanks! Congrats on your first merged PR! 🎉

[gilzoide]

Thanks for making up some tests to it! I had some trouble running tests on my machine (can't remember exactly what they were now), so I opened the PR without any and left them for later. Aaaand totally forgot about it 😅

[dsnopek]

Cherry-picked for 4.1 in PR #1281