Bug 1965855 [wpt PR 52476] - Add MutationObserver tests for <selectedcontent>,

Automatic update from web-platform-tests
Add MutationObserver tests for <selectedcontent>

These were requested here:
whatwg/html#10633 (comment)

There are failing test expectations added for the virtual test suites
where CustomizableSelect is disabled, where the <selectedcontent>
element is not enabled.

Change-Id: Ib2906a5c8d70ae5ea5cccb6d899c79b29ea45e5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6471702
Commit-Queue: Joey Arhar <jarharchromium.org>
Reviewed-by: Dominic Farolino <domchromium.org>
Cr-Commit-Position: refs/heads/main{#1458866}

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wpt-commits: 23b9c427c9115a1b40b95e4e4099c353a985c958
wpt-pr: 52476

Differential Revision: https://phabricator.services.mozilla.com/D249370

UltraBlame original commit: c92c496cf3b9513fb0279fdb8afa468388d866ff

Author: marco-c