Since the Parsoid DOM doesn't include TOC information, by design, we should go through all the parser tests which mention __TOC__ and make them Parsoid-compatible. Either (1) making them PHP-only, or (2) adding trivial html/parsoid clauses, self-documenting that Parsoid doesn't emit TOC info, or (3) adding some sort of "TOC normalization" to our normalization utility, to remove TOC markup.
(Note that there's also an issue with empty <p></p> being generated by the PHP TOC, see T92892, which might complicate option 3.)