[test-error] PayloadFormat.MultilineMultipleCueTextTag incorrect node kind assertions

This test asserts that the head node's kind is Node::Underline, which should always be false, as far as I understand it. The subsequent tests on child nodes are never called because the assertion will fail, but these would also be incorrect.

Test needs to be updated to check child of Head rather than the head node itself. (And subsequently, child of child of Head, and so on...)