Allow more-descriptive section names for parserTests

Allow more-descriptive section names for parserTests

This patch allows `!!wikitext` (for `!!input`) and `!!html` (for
`!!result`). This is more in line with what those sections actually
contain and closer to the semantics of how they are used.

The old names are accepted as aliases to accomodate parser tests to
provide a migration path for extensions and other users of the
parser tests framework.

In addition to `!!html`, this patch also accepts `!!html/*` and the
more-specific `!!html/php`.  This allows tests to include a number
of different "outputs" for a given wikitext input, for example
`!!html/parsoid` and `!!html/php`.

Co-authored-by: C. Scott Ananian <cscott@cscott.net>
Co-authored-by: Arlo Breault <abreault@wikimedia.org>
Change-Id: Ie4e68960ca7c352af495ebb59ba83488935a44c4