Per [spec](https://dom.spec.whatwg.org/#concept-node-insert), adoption
of new node should be done while inserting the node. This patch moves
the call site of `adopt` to inside `insert` to match it.
It also rewrites some existing code to better match the spec without any
behavioral changes.
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Signed-off-by: Xiaocheng Hu <xiaochengh.work@gmail.com>
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
