Auto merge of #17480 - asajeffrey:wpt-reenable-iframe-append-to-child-document, r=emilio

Re-enabled iframe-append-to-child-document.html

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Re-enabled a WPT test. Don't get too excited, the test FAILs.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15196
- [X] These changes do not require tests because it's re-enabling a test.

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