LibWeb+Tests: Update layout before accessibility tree dumps

The accessibility tree builder consults layout nodes while applying
inclusion and exclusion rules. DevTools can request an accessibility
tree after style or layout has been dirtied, which made those
layout-node lookups trip the stale-layout verification.

Update layout before serializing the accessibility tree, matching the
DOM tree dump path. Add an internals regression test that dirties
layout before requesting an accessibility dump.