ParsoidParser: ensure magic variable expansion uses pageLanguageOverride
This patch adds tests for the caching fix in
Ie76020dc4fa3545f827e1674051530b479f01f31, but these tests also revealed
that the recursive invocation of the legacy parser to expand magic
variables like {{PAGELANGUAGE}} wasn't using the pageLanguageOverride,
aka ParserOptions::getTargetLanguage().
The page language override is used when parsing new context which
doesn't currently exist in the database and therefore doesn't have a
page language set by its title (which doesn't yet exist).
Bug: T376783
Follows-Up: Ie76020dc4fa3545f827e1674051530b479f01f31
Change-Id: If6fe7cf00be6e78ef46181b17f01138383e95e46