HTML API: Fixes for issues discovered while fuzzing.

[dmsnell]

Trac ticket: Core-65372

Fuzz-testing was performed against the HTML API for finding edge cases
that might be broken in the existing parsing code. A few issues were
discovered with HTML normalization and warnings from out-of-bounds
string reads.

This patch contains new tests catching regressions on these behaviors
and adds fixes for the discovered issues.

A special-case for FORM closing tags inside TABLEs was added after
investigation turned up that many normalization issues stem from this
single issue, where next_tag() would already fail to proceed, but the
normalization was continuing with already-created virtual tokens. This
special-case should be investigated as more support is added to the HTML
API to ensure that it couldn’t be removed for more robust core code.

[github-actions]

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[github-actions]

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