Add wgAllowExternalImages option to parser tests.

This allows us to test whether the parser properly respects the
$wgAllowExternalImages option.

I also renamed the 'rawhtml' option to wgRawHtml so that parser test
options which set wiki configuration variables have consistent names.

Bug: 51092
Change-Id: I6c453b3e366cf775d8eef2dcbde09fcfa7027125
Merge "Add wgAllowExternalImages option to parser tests."