Improve test coverage for ApiParse

Improve test coverage for ApiParse

Also removed a sketchy-looking usage of ?: with a string in
ApiParse.php.  In this case I think it was fine, because it would only
cause a bug if a page's display title was '0' but its actual title was
not '0', which is only possible if $wgRestrictDisplayTitle is false,
which is broken by design anyway and I don't think is worth testing.
But ?: used for something that should be interpreted as a string is
generally not a good idea.

One bug fixed: an error message that used an undefined variable.

Depends-On: Id0e6184aff8f9d7e8f32558e1de14faa0168cc1d
Change-Id: I0904bff0f9d80892d0db2ebb590c24fb862f2418