Add missing tests for joint iteration proposal

[ojuschugh1]

Add missing tests for joint iteration proposal

Adds a few tests that were missing from the Iterator.zip and Iterator.zipKeyed coverage:

• Each call to next() should return a fresh array/object, not reuse the same reference
• In strict mode, when all iterators finish at the same step, the remaining ones still get stepped to confirm they're done, and the iterator completes normally instead of throwing

Closes #4112