[soft navs] Add smoke test for DOM modification support.

For now, this tests that we support Element.innerHTML, Node.appendChild, and Node.insertBefore, Document.importNode, Document.adoptNode, and template element.

This isn't a fix for the bug mentioned here, but hopefully some baby steps towards it. :-)

Bug: 328783345
Change-Id: I4be5de236acc8cf7da37a665b26c77f6f7f218ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532832
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Michal Mocny <mmocny@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459667}