No interlanguage links after local interwiki prefixes

This was noticed on enwiki after w: was marked as a local interwiki prefix
there. Links like [[w:de:Foo]] ought to act like [[:de:Foo]], not
[[de:Foo]].

Also adding a number of additional parser tests related to interwiki links.

Bug: 68085
Change-Id: If39af06edb4af2da85c9bcf43df7088181809fcf