PHP ParserTests adds three pseudo-extensions to verify extension tag parsing.

We should implement these in Parsoid, in order to similarly verify extension tag parsing. (As in PHP land, they ought to be installed only when running ParserTests.)

They would also serve as good living documentation of how to write a Parsoid extension-parsing module. The <gallery> extension has been wanting this for a while; Parsoid should really be extensible in this way.

(Also, the PHP <tag> implementation should use JSON serialization instead of PHP var_dump, so the Parsoid implementation doesn't have to implement a var_dump clone.)