[LiveHTML] Add more tests

This adds a few more tests for cases of typing character-by-character, as well as a test for "reordering" children.

Doesn't seem like that many tests :), but it does also add some infrastructure for making it easier to add new "character-by-character" tests, so we can do more of that easily in the future. It also makes it easier to create tests from simple strings.

Luckily, all these tests passed without any changes to the algorithm.

@dangoor