Tests for ItemPage.iterlinks

These tests use a fabricated sitelinks file, which makes testing easier as
we no longer depend on the actual WikiData data.

ItemPage.get() was adapted: its parent already prevents getting data from WD
if _content exists, and its parent needs to parse _content too. It's much easier
to mock data when we can just set _content and call .get().

Per Legoktm's comments, WD functionality from page_tests has been moved into
this file. For this, assertType(self, obj, cls) had to be moved from
TestPageObject to the new superclass PywikibotTestCase.

Change-Id: I5cd0e235db5bb0792d39178ee0c9e097fb053823
Merge "Tests for ItemPage.iterlinks"