Augment getHTML testing to include light DOM and inner element

Two cases were not being tested:
 1. Serializing an element that has both shadow DOM and also
    light DOM content.
 2. Serializing the *inner* HTML of an element that itself has
    a shadow root (which returns just the `<template>`).

It also turns out that in case 1, Blink was serializing the light
DOM children first, before the shadow `<template>`. This CL
corrects that.

Bug: 41490936
Change-Id: Iae1dc861d78e829a9305132e3f9dbb6fbcc467d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5421132
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283391}