Run some parser tests with tidy.

Run some parser tests with tidy.

Note that the old parser tests helper function `tidy()` never actually did
anything, since $wgUseTidy was forced to `false` in the parser test setup.
Remove this unused code, and replace it with our new tidy support.

Allows new parser test sections: 'html+tidy' denotes "tidied" HTML (open
tags closed and other fixups to original wikitext markup) which should be
applicable to any parser.  'html/php+tidy' is output specific to the PHP
parser with tidy turned on.  The Parsoid backend will use the 'html/parsoid'
section if present, but if it is not present it will fallback to first the
'html+tidy' section, and if that is missing the 'html' section.

Note that 'tidy' has a large number of open bugs (see
https://bugzilla.wikimedia.org/show_bug.cgi?id=2542 ) and so in some cases
we deliberately do *not* use 'html+tidy' or 'html/php+tidy' clauses, in
order to avoid documenting broken output.  In these cases, there is no
broken HTML in the PHP parser output, and so (in theory) the 'html' and
'html+tidy' sections would be identical (that is, if tidy didn't have
bugs).

Change-Id: Iba45f38774b221522dc3b6ae2d1312fb79f8f41f