Fix typo in template tag for Secure DNS setting

Previously crrev.com/c/5784654 added the new HTTPS-First Mode
settings, which updated the Secure DNS setting to be set up slightly
differently depending on whether the new HTTPS-First Mode settings
were enabled or not, using multiple "dom-if" templates (as a sort of
"if-else"). The "else" branch had a typo on the tag (`<template`)
which meant the dom-if property was never being applied.

This also adds regression tests to verify that only one or the other of
the Secure DNS setting variants is shown in each configuration (using
added `#secureDnsSettingOld` and `#secureDnsSettingNew` element IDs).

Fixed: 365884462
Change-Id: I597b7a63a9bf7eabc427472cab570ad2d565e4b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5857984
Reviewed-by: Rainhard Findling <rainhard@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1354920}