Bug 1857274 [wpt PR 42364] - Add test for lang attribute and shadow t…

…rees., a=testonly

Automatic update from web-platform-tests
Add test for lang attribute and shadow trees. (#42364)

This tests the lang attribute changes in whatwg/html#9796.  It replaces
the lang attribute tests from #29820.

Co-authored-by: Anne van Kesteren <annevkannevk.nl>
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wpt-commits: 3538a9f3031846a83f95f9c514388fc4f35437fc
wpt-pr: 42364

UltraBlame original commit: d4a31b5d4e9c6e85c5d6072481fd42108b885857