Auto merge of #29902 - Loirooriol:clearance-prevents-margin-collapse2, r=mrobinson

Fix interaction of clearance with margin collapse

In #29897 I did the simple naive thing, but it wasn't entirely correct. This patch tries to address the problems. In particular:

 - Clearance should prevent margins from collapsing through if it happens between them, as opposed to on the element that owns them.
 - The margins of an element with clearance can still collapse through, and collapse with other siblings as normal, but the resulting margin can't collapse with the bottom margin of the parent.

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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 #29896
- [X] There are tests for these changes

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